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NATL INST OF STANDARDS & TECH R.I.C.

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,. ,. „ ^V^P^^f^^^I^ r«n ndards and Technology /Guidelines for Implementing the new rep ^J

QC100 .U5753 N0.1263 1989 C.I NIST-PUB-R NIST

PUBLICATIONS R^-^^^ENCE

NIST Technical Note 1263

Guidelines for Implementing the

New Representations of the Volt and Ohm

Effective January 1, 1990

Norman B. Belecki, Ronald F. Dziuba, Bruce F. Field, and Barry N. Taylor

QC

100

U5753

#1263

1989
NATIONAL INSTITUTE OF STANDARDS &

TECHNOLOGY

r^"

Research Infonnation Center

he National Institute of Standards^||^j^p<^tta^jv|k/S3^l3€^99^ an act of Congress on March 3, 1901. The Institute's overall goal is to strengthen and advance the Nation's science and technology and

facilitate their effective application for public benefit. To this end, the Institute conducts research to assure international competitiveness and leadership of U.S. industry, science and technology. NIST work involves development

and transfer of measurements, standards and related science and technology, in support of continually improving

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physical and chemical measurement throughout the Nation's scientific community, industry, and commerce; provides advisory and research

services to other Government agencies; conducts physical and chemical

research; develops, produces, and distributes Standard Reference

Materials; provides calibration services; and manages the National

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Provides technology and technical services to the public and private

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' Located at Boulder, CO, with some elements at Gaithersburg, MD.
NIST Technical Note 1263

Guidelines for Implementing tlie

New Representations of ttie Volt and Ohm

Effective January 1, 1990

Norman B. Belecki, Ronald F. Dziuba, Bruce F. Field, and Barry N. Taylor

Electricity Division

National Institute of Standards and Technology

Gaithersburg, MD 20899

June 1989

^^t<*T o'-co^ NOTE: As of 23 August 1988, the National Bureau of 1^ V^ \ standards (NBS) became the National Institute of « . ^ . ^ Standards and Technology (NIST) when President

.,i.':i^.

*>> ^^^ Jf^

U.S. Department of Commerce

Robert A. Mosbacher, Secretary

National Institute of Standards and Technology

Raymond G. Kammer, Acting Director

Reagan signed into law the Omnibus Trade and

Competitiveness Act.
National Institute of Standards

and Technology

Technical Note 1263

Natl. Inst. Stand. Technol.

Tech. Note 1263

72 pages (June 1989)

CODEN: NBTNAE

U.S. Government Printing Office

Washington: 1989

For sale by the Superintendent

of Documents

U.S. Government Printing Office

Washington, DC 20402
Abstract

This document provides general guidelines and detailed instructions on how to bring

laboratory reference standards of voltage and resistance and related instrumentation

into conformity with newly established and internationally adopted representations of

the volt and ohm. Based on the Josephson and quantum Hall effects, respectively, the

new representations are to come into effect worldwide starting on January 1, 1990.

Their implementation in the U.S. will result in increases in the values of the national

volt and ohm representations maintained at the National Institute of Standards and

Technology (NIST, formerly the National Bureau of Standards or NBS) of 9.264 parts

per million (ppm) and 1.69 ppm, respectively. The resulting increase in the value of the

U.S. representation of the ampere will be 7.57 ppm and in the U.S. electrical representation of the watt, 16.84 ppm. Also discussed are the effects on electrical standards of

the January 1, 1990, replacement of the International Practical Temperature Scale of

1968 by the International Temperature Scale of 1990, and of the January 1, 1990, approximate 0.14 ppm decrease in the U.S. representation of the farad.

Scope

This document and the adjustments described in it apply primarily to standards, in- struments, and test equipment used for measurements of voltage or for voltage cali- brations at the 100 part per million (ppm) level of uncertainty or better, and to stan- dards and instruments used for resistance measurements or calibrations at the 20 ppm

level of uncertainty or better. Other types of electrical measurements possibly affected are those of power and energy at the 170 ppm level of uncertainty or better, alternating current and voltage measurements inasmuch as they are based on dc standards (100

ppm or better), and capacitance measurements at the highest levels of accuracy (2 ppm

or better). Instruments and standards having uncertainty tolerances greater than these

levels will not be affected.

m
i
Executive Summary

By international agreement, starting on January 1, 1990, the national standards laboratories of most major industrialized countries will put into place new

representations of the volt and ohm based, re- spectively, on the Josephson and quantum Hall ef- fects and which are highly consistent with the In- ternational System of Units or SI. Here, the volt is the

SI unit of electromotive force and electric potential

difference. In the past, 'legal volt', 'as-maintained

volt', 'national unit of voltage', laboratory unit of

voltage', and other similar terms were commonly used

to indicate a 'practical unit' for expressing measure- ment results. To avoid misunderstanding, in these

Guidelines the word unit is not used in this context; the expression representation of the volt and variations

thereof are used in its place. The situation for the

ohm and resistance is strictly analogous.

Also by international agreement, starting on Jan- uary 1, 1990, the national standards laboratories of

countries that do not have Josephson and quantum

Hall effect reference standards are requested to maintain their own national representations of the volt

and ohm so as to be consistent with the new in- ternationally agreed-upon representations. This consistency can be achieved through periodic comparisons with a laboratory that does have Josephson and

quantum Hall effect standards. As a consequence,

starting on January 1, 1990, the previous significant

differences among the values of some national volt

representations, among the values of many national

ohm representations, and the differences between the

values of most national volt and ohm representations

and the SI should no longer exist. Implementation of the new volt and ohm representations in the U.S. requires that on January 1, 1990,

the value of the present national volt representation

maintained by the National Institute of Standards

and Technology (NIST, formerly the National Bureau

of Standards or NBS) be increased by 9.264 parts per

million (ppm) and that the value (on this date) of the

national ohm representation be increased by 1.69 ppm.

The resulting increases in the present national repre- sentation of the ampere and in the present national

electrical representation of the watt will be 7.57 ppm

and 16.84 ppm, respectively.

The required January 1, 1990, changes in the U.S.

volt and ohm representations are sufficiently large

that literally thousands of electrical reference stan- dards, electrical measuring instruments, and electronic

systems throughout the Nation will have to be adjusted in order to bring them into conformity with the

new representations. (Similar adjustments will have

to be made in other countries as well.) Examples of

such equipment are groups of standard cells in ther- moregulated enclosures and oil baths; solid-state volt- age references and standards; wire-wound and metalfilm standard and precision resistors; high precision

digital voltmeters, multimeters, programmable

sources, calibrators, and standard watthour meters; automatic test equipment or ATE; and avionics sys- tems.The purpose of this document is to provide general guidelines, detailed instruchons, and helpful background information for those individuals and organizations that must deal with or are in some way af- fected by the January 1, 1990, changes in the U.S. volt

and ohm representations. These include metrologists, standards and calibration laboratories and their

personnel, equipment managers, quality assurance

personnel, instrument manufacturers and rental companies, engineers involved in manufacturing, production, and testing, ATE users and designers, and

those involved with procedures and software.

Because the 9.264 ppm increase in the U.S. volt representation is 19% of a 50 ppm tolerance, voltage

standards and related instrumentatton are especially

affected. Inasmuch as these Guidelines give technical

advice, point out potential problems, and identify

precautions to be taken, they are essential reading for those individuals who are in any way involved with

or are responsible for such hardware. To assist NIST

in the preparation of the Guidelines, National Confer-
Executive Summary (continued)

ence of Standards Laboratory Ad Hoc Committee 91.4 "Changes in the Volt and Ohm" was formed with

broad representation from the electrical metrology

community. The committee members helped to determine its contents, provided useful comments, and

served as a knowledgeable sounding board for ideas.

Also, starting on January 1, 1990, the International

Temperature Scale of 1990 (ITS-90) will supersede

the International Practical Temperature Scale of 1968

(IPTS-68). The new temperature scale is much more

consistent with thermodynamic temperature than is IPTS-68, which has been found to deviate significantly from thermodynamic temperature in certain

temperature regions. The introduction of ITS-90 in

place of IPTS-68 will affect electrical reference stan- dards such as standard cells and resistors since they

are assigned values at particular temperatures. These

Guidelines give detailed instructions on how to deal

with changes in electrical standards introduced by

ITS-90.

Moreover, on January 1, 1990, the U.S. representation of the farad will be decreased by about 0.14 ppm

to bring it into better agreement with the farad as defined in the SI. The effect of this change is also discussed.

An important consequence of adjusting standards

and instruments throughout the world so that they

conform with the new internationally adopted rep- resentations of the volt and ohm based on the Jo- sephson and quantum Hall effects is improved uniformity of electrical measurements worldwide and

their consistency with the SI. The resulting signifi- cant benefits to commerce, industry, and science

throughout the world are expected to far outweigh

the costs of making the adjustments.

VI
TABLE OF CONTENTS

page

Abstract iii Scope iii Executive Summary v

1. Introduction 1 1.1 NCSL Ad Hoc Committee 91 .4 1 1.2 The SI Units of Electromotive Force and Resistance and Their Representations 1 1.3 Josephson and Quantum Hall Effect Reference Standards of Voltage and Resistance 2

1.4 The Internationally Adopted Values of the Josephson and von Klitzing Constants 2

1.5 Symbols for the Old and New U.S. Volt and Ohm Representations 3

2. General Guidelines 4

2.1 Identifying Standards and Instruments Affected 4

2.2 Do's and Don'ts 5

2.2.1 Full recalibrations may not be necessary 5

2.2.2 Annotate control charts 6

2.2.3 Document! Document! Document! 6

2.2.4 Use the logo 6

2.2.5 Annotate calibration reports 6

2.3 Understanding the Role of a Unit 6

3. Change in the U.S. Volt Representation 7

3.1 Changing a Local Representation Based on the Josephson Effect .8 3.2 Changing a Local Representation Based on Standard Cells 8

3.3. Changing a Local Representation Based on Solid-State References 9

3.4 Updating Control Charts 10

3.5 Do Not Correct Pre-1990 Data 12

3.6 Effect of the International Temperature Scale of 1990 on Standard Cells 12

4. Change in the U.S. Ohm Representation 13

4.1 Conversion Factor 13

4.2 Examples 14

4.3 Updating Predicted Values and Control Charts 15

4.3.1 Predicted values 15

4.3.2 Control charts 16

4.4 Effect of the International Temperature Scale of 1990 on Standard Resistors 18

4.5 Summary 19

5. The Reporting of Calibration Results by NIST 20

6. Adjustment of Instrumentation 23

6.1 Voltage Instrumentation 23

6.2 Resistance Instrumentation 24

6.3 General Considerations Regarding Adjusting Instruments 24

6.3.1 Adjust instruments without using standards at your own risk 24

6.3.2 Where should the adjustments be made? 25

vu
TABLE OF CONTENTS (continued)

page

7. Other Quantities 26

7.1 Direct Current 26

7.2 Alternating Voltage and Current 26

7.3 Capacitance and Inductance 27

7.4 Power and Energy 27

8. Transducers 28

9. References 28

Apf>endix 1 Membership of the NCSL Ad Hoc Committee 91.4 30

Appendix 2 Concerns of NCSL Ad Hoc Committee 91.4 about Logistics and Management

Issues Stemming from Changes in the Unit Representations 32

Appendix 3 Reprints 36

New Internationally Adopted Reference Standards of Voltage and Resistance, B. N. Taylor

News from the BIPM, T. J. Quinn

Report on the 17th Session of the Consultative Committee on Electricity, B. N. Taylor

History of the Present Value of 2e/h Commonly Used for Defining National Units of

Voltage and Possible Changes in National Units of Voltage and Resistance, B.N. Taylor

vm
Guidelines for Implementing the New Representations of the Volt and Ohm

Effective January 1, 1990

Norman B. Belecki, Ronald F. Dziuba, Bruce F. Field, and Barry N. Taylor

1. Introduction

By international agreement, starting on January 1, 1990, new practical representations of the volt and

ohm based on the Josephson and quantum Hall effects, respectively, will come into effect worldwide.

It is the principal purpose of this document to provide detailed instructions on how to bring laboratory

reference standards of voltage and resistance and re- lated instrumentation into conformity with the new

representations. This introductory portion of the

Guidelines contains background information which

should help the reader better understand the more

practical sections. However, it is highly abbreviated

since such information is available in a number of

publications[l-8].^ For the convenience of the reader,

those articles that are likely to be of most use (refs.

[1, 3, 4, 5]) are reprinted in Appendix 3.

1.1 NCSL Ad Hoc Committee 91.4

This document would not have been possible without the active assistance of the members of National

Conference of Standards Laboratories (NCSL) Ad Hoc

Committee 91.4 "Changes in the Volt and Ohm" (the

Committee members and their affiliations are listed in Appendix 1). Drawn from a large number of or- ganizations spanning the range from DoD (Department of Defense) standards laboratories to manufacturers of high precision electrical measuring instruments, their varied backgrounds helped to ensure

that the needs of all segments of the U.S. electrical metrology community were addressed by these Guidelines. Through dedicated Committee meeting atten- dance and participation as well as by correspondence,

they provided much valuable information and many

critical comments. Indeed, Appendix 2 directly reflects the views of Committee members regarding

management and logistics issues. The Committee es- pecially served as an extremely knowledgeable sounding board for ideas proposed by the members from

NIST (formerly the National Bureau of Standards or NBS). The authors thank all of the members of 91.4

References are listed on page 28.

for their many contributions to this document but, of

course, take full responsibility for any omissions or

errors.

1.2 The SI Units of Electromotive Force

and Resistance and Their Representations

The International System of Units [9], abbreviated

SI, serves as a basis for the promotion of long-term,

worldwide uniformity of measurements. In the SI, the unit of electromotive force or emf (symbol E) and

electric potential difference (17) is the volt (V). The

unit of resistance (R) is the ohm (Q). In practice, the

volt and ohm, which occasionally may be referred to in the literature as the absolute volt and ohm, may be

realized in a number of ways. These include comparing electrical power with mechanical power (for the

volt) using a force balance [10], and resistance with

impedance (for the ohm) using a calculable capacitor

[11,12].

However, commercial, industrial, and scientific re- quirements for the long-term repeatability and worldwide consistency of measurements of emf/electric

potential difference and resistance often exceed the

accuracy with which the SI units for such measurements, the volt and the ohm, can be readily realized.

To meet these severe demands, it has become neces- sary to establish representations of the volt and ohm

that have superior long-term reproducibility and con- stancy compared with the present direct realizations

of the volt and ohm themselves.

Since the phrase representation of the volt may be

unfamiliar to some readers, it requires explanation.

In the past, legal volt', 'as-maintained volt', 'nahonal

unit of voltage', laboratory unit of voltage', 'practical

realization of the volt', and other similar terms were commonly used to indicate a 'practical unit' for ex- pressing measurement results. However, to avoid

possible misunderstanding, it is best not to use the

word unit in this context. The only unit of emf and

electric potential difference in the SI is, of course, the

volt. In these Guidelines, the expression representation
of the volt and variations thereof are used in its place.

The situation for the ohm and resistance is strictly

analogous.

Historically, laboratory volt representations have

been based on electrochemical standard cells. However, starting in the early 1970's, it became possible

for national standards laboratories to base their volt

representation on the Josephson effect in order to avoid the well-known problems associated with the

emf's of standard cells, for example, their variation

with time (i.e., drift), severe dependence upon temperature, and occasional unpredictable abrupt

changes.

On the other hand, most national standards laboratories have continued to base their ohm repre- sentation on the mean resistance of a particular group

of wire-wound standard resistors. Because such arti- fact standards age, the various national ohm repre- sentations differ significantly from each other and the

SI ohm, and some are drifting excessively. Electrical metrologists therefore welcomed Klaus von Klitzing's

1980 discovery of the quantum Hall effect (QHE) since

it promised to provide a method for basing a repre- sentation of the ohm on invariant fundamental constants in direct analogy with the Josephson effect. The QHE clearly had the potential of eliminating in a

relatively straightforward way the problems of

nonuniformity of national ohm representations, their

variation in time, and their inconsistency with the SI.

13 Josephson and Quantum Hall Effect

Reference Standards of Voltage and

Resistance

The Josephson effect is characteristic of weakly

coupled superconductors when cooled below their

transition temperatures. An example is two thin films

of superconducting lead separated by an approximately 1 nm-thick thermally grown oxide layer.

When such a Josephson junction is irradiated with

microwave radiation of frequency /, its current vs

voltage curve exhibits steps at highly precise quantized Josephson voltages Uy The voltage of the nth

step Lij(n), n an integer, is related to the frequency of

the radiation by

L7j(n) = nflKy (1.1)

where Ky now termed the Josephson constant, is a

universal quantity independent of experimental variables, such as type of superconductor and irradiation

frequency, to very high precision. (It follows from eq.

(1.1) that Kj is the frequency-to-voltage quotient of

the n = 1 step.) Indeed, theory and experiment indi- cate that Kj is equal to the invariant quotient of fundamental constants lajh, where e is the elementary charge

and h is the Planck constant. Numerically, K. is about

483 598 GHz/V. Because quantized Josephson volt- ages depend only upon a readily measured frequency

and invariant fundamental constants of nature, a volt

representation based on the Josephson effect has none

of the problems characteristic of standard cells indicated above.

The QHE is characteristic of certain high-mobility

semiconductor devices of standard Hall-bar geometry

when placed in a large applied magnetic field and

cooled to a temperature near one kelvin. For a fixed current I through a QHE device there are regions in

the curve of Hall voltage vs gate voltage, or of Hall

voltage vs magnetic field depending upon the device,

where the Hall voltage U^^ remains constant as the

gate voltage or magnetic field is varied. These re- gions of constant Hall voltage are termed Hall plateaus. Under the proper experimental conditions, the

quantized Hall resistance of the zth plateau R^i), defined as the quotient of the Hall voltage of the ith plateau to the current I, is given by

R^{i)=UJ.i}/I = R^/i, (1.2)

where i is an integer and R^^ is now termed the von

Klitzing constant after the discoverer of the QHE. (It follows from eq. (1.2) that Rj, is equal to the resistance

of the i = 1 plateau.) The von Klitzing constant has

been shown to be a universal quantity independent

of experimental variables, such as type of QHE device and plateau number, to high precision. Indeed,

theory and experiment indicate that Ry^ is equal to the

invariant quotient of fundamental constants h/ef^. Numerically, R^ is about 25 813 Q..

1.4 The Internationally Adopted Values of

the Josephson and von Klitzing Constants

At its October 1988 meeting [1,3], the International

Committee of Weights and Measures (CIPM) recom- mended that all national standards laboratories that base their representation of the volt on the Josephson

effect adopt the same conventional value of Ky namely.

^^,0 = 483 597.9 GHz/V (1.3)

exactly, where the subscript 90 indicates that this value

is to come into effect starting on January 1, 1990, and
not before. The CIPM also stated its belief that an

ideal volt representation based on the Josephson ef- fect and Kjgg will be consistent with the SI volt to within a one-standard-deviation (la) assigned uncertainty of 0.4 parts per million (ppm). Similarly, the

CIPM recommended that all national standards laboratories that choose to base their representation of the

ohm on the QHE use the same conventional value of

the von Klitzing constant, namely.

Rj^,,o= 25 812.807 Q; (1.4)

and stated its belief that an ideal ohm representation

based on the QHE and R^^g^ will be consistent with

the SI ohm to within an assigned la uncertainty of 0.2 ppm.

It should be recognized that the definitions of the

SI volt and ohm are not being changed; rather, more

accurate representations of them have been made

possible by improvements in measurement science.

1.5 Symbols for the Old and New U.S. Volt

and Ohm Representations

The U.S. volt representation maintained at NIST

has been based on the Josephson effect since July 1, 1972, and the U.S. ohm representation has been based on the mean resistance of groups of standard resis- tors since January 1, 1948.

For the purpose of explaining how to bring stan- dards and instruments calibrated in terms of these

pre-January 1, 1990, representations into conformity

with the new, post January 1, 1990, representations,

we shall use the following symbols defined as indicated:

V(NBS-72) — The U.S. representation of the volt

based on the Josephson effect maintained at NBS/NIST during the period July 1, 1972,

through December 31, 1989, using K. =

483 593.420 GHz/V(NBS-72) exactly as the

value of the Josephson constant [13]. (NIST was the National Bureau of Standards or NBS

prior to August 23, 1988.)

V(NIST-90) — The U.S. representation of the volt based on the Josephson effect to be maintained at NIST starting January 1, 1990, using

the new internationally agreed upon or conventional value of the Josephson constant,

Kj_g^ = 483 597.9 GHz/V exactly. Since K^_^

exceeds the previous value of K used by NBS/

NIST by 9.264 ppm, V(NIST-90) exceeds

V(NBS-72) by this amount.

Q(NBS-48), — The U.S. representation of the ohm

based on the mean resistance of groups of

standard one-ohm resistors maintained at NBS/NIST during the period January 1, 1948,

through December 31, 1989. The subscript t indicates that this ohm representation is time

dependent and thus has a unique value only

at the time t. Q(NIST-90) — The U.S. representation of the ohm

based on the quantum Hall effect to be maintained at NIST starting January 1, 1990, using

the internationally agreed-upon or conventional value of the von Klitzing constant,

^^^.^o = 25 812.807 a exactly. From quantized Hall

resistance measurements it is calculated that

Q(NIST-90) will exceed ii(NBS-^8),

1.69 ppm.

01/01/90 by

It must be emphasized that these symbols will be

used only in this document and for the explanatory

purpose given near the start of this section. When

rep)orting the results of NIST calibrations of client voltage and resistance standards in terms of the new

1990 volt and ohm representations (starting January

1, 1990), NIST will follow the recommendation of the

Consultative Committee on Electricity (CCE), affirmed

by the CIPM, and not use any distinguishing symbols on either unit symbols or quantity symbols [1, 3, 6]. That is, calibration results will be expressed in terms

of V and ^, not V(NIST-90) and Q(NIST-90); see sec. 5 for a detailed discussion.

The January 1, 1990, increases of 9.264 ppm and

1.69 ppm in the values of the U.S. volt and ohm

representations, respectively, will require the adjustment of literally thousands of electrical reference stan- dards, electrical measuring instruments, and electronic

systems throughout the U.S. in order to bring them

into conformity with the new 1990 volt and ohm representations. (Similar adjustments will have to made in other countries as well.) Examples of the type of

equipment affected are groups of standard cells in

themoregulated enclosures and oil baths; solid-state

voltage references and standards; wire-wound and

metal-film standard and precision resistors; high precision digital voltmeters, multimeters, programmable

sources, calibrators, and reference standard watthour

meters; automatic test equipment or ATE; and avionics systems.

It is the principal purpose of these Guidelines, es- pecially the remaining sections, to help those in- dividuals and organizations that must deal with such

equipment to accommodate to the new representations
of the volt and ohm in as efficient a manner as possible. Indeed, twenty years ago when there were far fewer voltage standards and related instruments and

systems in existence for which 10 ppm was at all significant, the January 1, 1969, decrease of 8.4 ppm

in the U.S. volt representation caused considerable

difficulty and many individuals dealt with it incorrectly. These Guidelines are a direct result of the 1969

experience when no such detailed document was

available.

2. General Guidelines

To repeat, on January 1, 1990, the volt and ohm rep- resentations in the United States will change. This

will immediately create an incompatibility between

your volt and ohm standards and those of NIST and

the rest of the world unless you act to bring yourself into compliance with the changes. The changes are +9.264

ppm and +1.69 ppm for the volt and ohm representations, respectively, or

V(NIST-90) = 1.000 009 264 V(NBS-72)

and

t2(NIST-90) = 1.000 001 69 QCNBS-^S)^^/^!/,^

.

Accordingly, those receiving calibration reports

from NIST dated January 1, 1990, or later will find the

reported values for voltage and resistance standards

to be 9.264 ppm and 1.69 ppm smaller, respectively,

than they would have been had no change taken

place.

Because of the relatively small size of the changes,

most instruments will not be affected; however, high

accuracy standards and test equipment will be. The

following section gives recommendations about how

to determine what class of instruments should be

adjusted.

2.1 Identifying Standards and Instruments

Affected

As noted above, the National Conference of Standards Laboratories (NCSL), in cooperation with NIST,

formed Ad Hoc Committee 91.4, "Changes in the

Volt and Ohm," to help the metrology community come into compliance with the new volt and ohm

representations. The committee has wide-spread

technical and management representation from in- dustry and government and this document reflects the thinking of its membership.

The conservative rule recommended by NCSL Ad

Hoc Committee 91.4 for use in determining the circumstances for which adjustments to test equipment

and instrumentation should be made is:

• if any required measurement uncertainty is less than or equal to five times the magnitude of the

change, adjustment of the instrument should be

made as soon as practical on or after January 1, 1990;

• if required measurement uncertainties are between

five and ten times the magnitude of the change,

this constitutes a "gray" area and the situation

must be assessed on a case-by-case basis; and

• if all required measurement uncertainties are

greater than ten times the magnitude of the change, no adjustment need be made.

However, since standards are the beginning of the

calibration chain, the recommendation regarding their adjustment is more conservative. The values of all voltage standards having accuracies of 100 ppm or better should be adjusted to bring them into compliance

with the changes. For resistance, the values of all standards with accuracies of 20 ppm or better should

be adjusted. The adjusted values should be used beginning January 1, 1990, the date that the changes

legally take effect. A delay in implementing the

changes, even for secondary standards, creates the

possibility that instruments recalibrated after the date

of the changes might unintentionally be calibrated in terms of V(NBS-72) or Q(NBS-48),

.

Table I describes the application of the Ad Hoc

Committee rule to manufacturers' specifications for instruments and test equipment. In each case, the

magnitude of the change was rounded up to the next

highest whole number (in ppm).

It follows from this rule that the instrument accu- racy required and its ratio to the magnitude of the

change must be determined in order to decide if the

instrument should be adjusted. The manufacturer's

specifications may be used for this purpose. They

generally give instrument accuracies to be expected

for several different time intervals; for example, digi-
TABLE I Manufacturers' Specified Accuracies (MSA)

IN PPM

Must Adjust

"Gray" Area

Forget It

Volt

MSA < 50

50 < MSA < 100

MSA > 100

Ohm

MSA < 10

10 < MSA < 20

MSA > 20

tal voltmeter accuracies are commonly given for 24

hours, three months, and one year. Typically, one set of specified accuracies is chosen based on the desired

recalibration interval. The 91.4 recommendation is to use this chosen accuracy to make the adjustment decision for normal use situations, but the manufacturer's 24-hour accuracy specification should be considered for use in situations of special criticality, when

the test equipment is frequently removed for other uses under uncontrolled circumstances, or when ex- tremely conservative practice is desired.

Since 9.264 ppm is significant relative to accuracies of 50 ppm or better, all voltage instrumentation

normally calibrated to this level of accuracy should

be adjusted as soon as practical on or after January 1, 1990. However, it is clear that a change of 9.264 ppm

will not have a significant effect on the results of measurements made using an instrument whose accu- racy specifications are at the 100 ppm level or worse. No effort should be made to adjust or recalibrate such

instrumentation other than that normally made, i.e., the change can be ignored.

Instruments and test equipment whose calibrated

accuracy for voltage measurements lies between 50

and 100 ppm fall into a "gray" area. The decision of

whether to adjust immediately or wait until the in- strument is due for recalibration must be made on a

case-by-case basis depending on the use to which the

particular instrument or device is put, its interval

between recalibrations, and particularly on the criti- cality of the measurements for which it is used.

The increase in the value of the ohm representation of 1.69 ppm is much smaller. Therefore, in the

case of multimeters and calibrators with resistance

capabilities, those with calibration sp)ecifications of 20

ppm or greater can safely be ignored (i.e., not adjusted until their regular calibration due date); those

with specifications of 10 ppm or better should be

adjusted as soon as practical; and the range in between again constitutes a "gray" area where each

case should be individually decided. The population

of instrumentation requiring adjustments for resis- tance measurements is extremely small.

The increase in the ampere representation will be

7.57 ppm. There are no standards for current per se; voltage and resistance standards are used to measure

currents. A limited number of instruments are ca- pable of current measurements at the 80 ppm level.

If, however, adjustments must be made to a digital

multimeter or a calibrator, the manufacturer's procedures should be followed.

The National Conference of Standards Laboratories, in cooperation with U.S. manufacturers of test equipment, is compiling lists of model numbers of

test equipment which should definitely be adjusted

and which should be considered for adjustment.

These lists will be obtainable by writing:

The National Conference of Standards

Laboratories

1800 30th Street, Suite 305B

Boulder, CO 80301

Telephone (303) 440-3339.

The lists will also be made available through

GIDEP, the Government Industry Data Exchange

Program. For further information write to:

Government - Industry Data Exchange Program

GIDEP Operations Center

Corona, CA 91720

Telephone (714) 736-4677.

2.2 Do's and Don'ts

2.2.1 Full recalibrations may not be

necessary

Keep in mind that full recalibrations are probably not

necessary. In a full recalibration, a large selection of

values over all ranges and functions is typically measured to determine if the instrument meets Sf)ecifications, and to adjust it if necessary. In the instruments

which will be most affected by the changes —dc volt- age calibrators, high-resolution programmable volt- age sources, precision digital multimeters, and a few

of the most accurate data converters — levels are set or readings made relative to one or more internal ref- erences, e.g., circuits based on Zener diodes in the case of dc voltage. Since the output voltage of a

calibrator or the reading of a meter is proportional to the output level of the internal instrument reference
no matter what the range, any adjustment made to the reference affects all other settings or readings

proportionally. Therefore, it may be possible to adjust an instrument to the new voltage scale without

fully recalibrating it. Similar thinking applies to other

functions such as resistance, ac voltage, and current. A number of manufacturers intend to provide

procedures to facilitate such adjustments to their

products. A list of references to such procedures will

be available through NCSL and GIDEP. In any event, manufacturers' instructions should be consulted to deter- mine if such an approach is feasible and then used to make

the adjustment.

2.2.2 Annotate control charts

Control charts are a necessity when state-of-theart standards are being maintained at the highest ac- curacy levels and a good idea in any case where

measurements are crucial to a company's quality- assurance efforts. These should be carefully annotated. In the case where differences among standards

are being plotted, the change to the new representations will not appear on the control chart. Explanatory notes on such a chart will help others to reconstruct the nature of the changes and when they were

made. This is especially important when there is a turnover in laboratory personnel or when problems

with the quality of calibration laboratory work are perceived by others. Where actual values of stan- dards are controlled and a single vertical scale used,

the graphs should show a downward discontinuity on January 1, 1990. If the discontinuity is upward, a mistake has been made. See sees. 3.4, 3.5, and 4.3.2 for specific recommendations on control charting.

2.2.3 Document! Document! Document!

Failure to record changes can lead to disaster!! In keeping with normal good laboratory practice, all changes in the laboratory should be documented. An

orderly sequence of adjustments should be planned,

beginning with the primary standards, and changes

should be recorded as they are made — not only on

the calibration records of the specific equipment in- volved, but in a laboratory log book as well. If a mistake is made, such a record can be invaluable in

determining the extent to which the population of

instrumentation is affected. At the end of the se- quence of adjustments to equipment within the laboratory, and before adjustments to clients' test equipment or instrumentation are made, the last item adFig. 1. NCSL logo for indicating equipment that is cali- brated or adjusted to the new unit representations

(black print on a green background).

justed should be checked directly against the primary

standards, if possible, to be sure the adjustments were made consistently, the correct sign was observed, etc.

2.2.4 Use the logo

NCSL Ad Hoc Committee 91.4 has devised a logo

for the purpose of marking standards, instruments,

and calibration rep)orts to indicate that the calibrations have been done in terms of V(NIST-90),

Q(NIST-90), their derivatives, or ITS-90. The logo is shown in Fig. 1. Stickers, 1.78 cm by 1.27 cm (0.7 in. by 0.5 in.), bearing the logo will be available through the National Conference of Standards Laboratories for a nominal fee. See sec. 2.1 for the address of NCSL.

2.2.5 Annotate calibration reports

Calibration reports are a source of data or correc- tions for standards and many types of test equipment. Users may rely on the report information.

Accordingly, it is important to annotate all calibration reports in which values are expressed in terms of

the new representations, or quantities derived from

them, for at least three years after the changes take

effect. This should be done by means of an explicit

statement such as: "The results presented in this re- port are expressed in terms of the NIST realization of

the new 1990 representation of the volt." or by affix- ing the logo in a prominent place on the report.

2.3 Understanding the Role of a Unit

It is extremely important to understand the role of a unit (or a representation of a unit) in a measurement, and what effect the change of a unit has on the
measurement, as a lack of such understanding can

easily lead to confusion and result in errors.^ Perhaps the key to understanding units is the real- ization that they are agreed-ufX)n fixed reference

quantities, which enable us to measure and describe

quantitatively the characteristics of objects, phenomena, and their behavior. The value of a physical

quantity is equal to a "label" times a unit, where the

"label" is simply a number. For example, the emf of a standard cell may be written as

emf = 1.018 123 V(NBS-72)

where the number 1.018 123 is its label and V(NBS-72)

its unit. The result of changing a unit, then, in the case of non-adjustable standards such as standard

cells is that the label must be changed to make it consistent with the new unit. In constrast, for adjustable standards, meters, and sources, one has the option of adjusting the value of the standard, reading, or output so that the label remains the same. The lack

of distinction between the units themselves and the

standards used to represent them, and between the

quantities being measured and the measurement re- sults or labels, are the primary causes of error when

changing units.

To illustrate the notion of labelling, consider the

calibration of a nominal 100-liter vessel. One approach to calibrating the vessel would be to repeatedly pour the contents of a one-liter graduated cylinder into the vessel, counting the number of liters —

including fractional pours —required to fill the vessel. Suppose the result is 100.03 liters. Now further

suppose that the unit of length, the meter, is increased

by 0.02% as the result of a technological breakthrough.

Since volume is defined in terms of length cubed, this causes an increase in the liter of 0.06%. Consequently,

the order goes out to destroy all graduated cylinders,

and new cylinders, larger by 0.06%, are issued to represent the liter. Since the volume of the vessel has not

changed but the volume of the cylinder used to cali- brate it is larger, the number of pours will have to decrease. In fact, it decreases by 0.06% and in the

new unit system the container has a capacity of 99.97

liters. Thus the unit has increased and the "label" of

the fixed-size vessel has decreased — not the true volume, but what one calls the volume, or what one

labels the volume to be. This is exactly the case in the

forthcoming changes in the volt and ohm representations — the units will increase and the assigned val- ues of fixed standards such as standard cells and

resistors will decrease.

Note that in the discussion above, all the changes

mentioned are proportional, that is, expressed as a percentage of the magnitude of the quantity discussed. The ratio of the "new" volume of any fixedcapacity container to the old volume is 0.9994. Also

note that actual recalibration of a container is not

necessary; one can simply multiply its label under the

old unit system by 0.9994 to obtain the new label. Nowhere does one actually have to use the new

graduated cylinders to measure the volume of any

previously measured container.

The following two sections discuss in detail how

to adjust or restandardize voltage and resistance stan- dards to be consistent with V(NIST-90) and

Q(NIST-90).

3. Change in the U.S. Volt Representation

From July 1, 1972, through December 31, 1989, the

U.S. volt representation, V(NBS-72), as maintained

by NBS (now NIST) has been based on the Josephson

effect using a value of the Josephson constant, K„ of

483 593.420 GHz/V(NBS-72). As a consequence there

has been no perceptible drift in V(NBS-72) during

that time. There did exist, however, small differSee sec. 1.2 for a discussion of the distinction between a unit and the

representation of a unit. On January 1, 1990, the representations of the volt and ohm in the United States will change. However, for illustrative purposes, in this section we equate this with a change in the unit; the

principle is the same for a change in the representation of a unit.

ences between the volt representations maintained at different national laboratories because they chose to use different values of K [1, 5]. As a result of the realignment of the national volt representations to be consistent with the SI volt, on

January 1, 1990, NIST will adopt a new value for Ky

namely, 483 597.9 GHz/V. This will introduce a one

time step increase in the NIST volt representation of

9.264 ppm and for clarity this new representation is referred to within this document as V(NIST-90). As

with V(NBS-72), there will be no significant drift in

V(NIST-90) after the change. Also starting on Janu-
ary 1, 1990, all NIST calibration reports for voltage

standards will report the values of client standards

based on V(NIST-90) using the CCE recommended

terminology. The terms 'U.S. legal volf, TsTIST volf

,

or 'V(NIST-90)' will not be used; all values will be re- ported in terms of the SI volt. However, all uncertainties will be reported as if the calibration results were expressed in terms of V(NIST-90), i.e., the uncertainty of V(NIST-90) with respect to the SI volt

will not be included. Thus, all uncertainties will be

essentially identical to the uncertainties for calibrations previously reported in terms of V(NBS-72). (For more detail see sec. 5 regarding NIST reporting of uncertainties.)

Standards laboratories that maintain a local volt

representation must "adjust" the value of their repre- sentation on January 1, 1990, to remain consistent

with the NIST representation. Since most high accu- racy volt representations are maintained using the Jo- sephson effect, saturated standard cells, or solid-state

voltage references, examples are given below for the

"adjustment" of these standards.

3.1 Changing a Local Representation

Based on the Josephson Effect

This is the easiest change to make; on January 1, 1990, start using the new value for K , namely,

483 597.9 GHz/V. This will produce a decrease in the

calibrated values of your voltage standards.

3.2 Changing a Local Representation Based on Standard Cells

The emf of a standard cell is not physically adjustable and thus one must correct the assigned value of

the emf to be consistent with the new value of JC.. This correction is computed as follows:

The change in the volt representation may be ex- pressed in equation form as

1 V(NBS-72) = 0.999 990 736 V(NIST-90). (3.1)

If e is the numerical value (i.e., label) of a standard cell based on V(NBS-72), the cell emf E is

E = e V(NBS-72), (3.2)

the numerical value times the unit. To express the

emf in terms of V(NIST-90), we replace V(NBS-72) in

eq. (3.2) with eq. (3.1), to obtain a corrected emf of

or.

E = e [0.999 990 736 V(NIST-90)]

E = 0.999 990 736e V(NIST-90).

The correction for the new representation is thus

accomplished by multiplying the previous numerical

value of the cell emf by the factor 0.999 990 736 to obtain the new corrected value. This is also equivalent to subtracting 9.264 ppm of 1.0181 V, or apIMPORTANT: Determining the January 1, 1990 value of a reference standard

Because V(NIST-90) becomes effective on Jan- uary 1, 1990, the volt correction must be applied to the January 1, 1990, value of your reference stan- dard. The number you use for this value depends on your current procedures. (The example below is for standard cells; the situation for solid-state refer- ences is exactly analogous.)

CASE 2 . If you assume the drift rate of your cells

is negligible and/or you always use the last value

of a cell emf from a calibration report until the cell

is recalibrated, the volt correction should be applied to the value on the last calibration report.

CASE 2. If you know the drift rate of your cells (or the drift rate of the mean of your group of cells) and your current practice is to reassign the values to the

cells to correct for the drift rate, you must adjust the

drift-corrected values, NOT the emf values from

the last calibration report. For example, suppose

you have a standard cell that was calibrated at NIST on April 1, 1989, with the calibrated value given as 1.01812166 V(NBS-72). Further, suppose that from

past history you know that the cell emf is decreasing at 1 |iV per year and you periodically assign

new values to the cell to account for this drift. Based on V(NBS-72) the value of the cell on January 1, 1990, would be:

1.018 121 66 V(NBS-72)

- (1 |j.V/year) x 0.75 years = 1.018 120 91 V(NBS-72).

The volt correction should then be applied to this calculated value.
proximately 9.43 |iV, from the previously assigned

cell emf.

In summary, to "correct" the value of a standard

cell, given its January 1, 1990, numerical value based on V(NBS-72) [see IMPORTANT on page 8] mulHply

it by 0.999 990 736 to obtain the value for January 1, 1990, based on V(NIST-90), e.g.,

1.018120 91 V(NBS-72)

X 0.999 990 736 V(NIST-90)/V(NBS-72) = 1.018 111 48 V(NIST-90).

After the correction is made, the assigned value of

the standard cell emf should be about 9.43 |iV smaller.

The change applies, of course, to all of the standard

cells in the laboratory, including those used as check

standards or working cells, as well as those considered to be the primary cells. The uncertainty of the

value remains the same.

3,3 Changing a Local Representation Based on Solid-State References

Unlike the emf of a standard cell, the voltage of a

solid-state reference can usually be adjusted. It is thus tempting to increase the output voltage physically by 9.264 ppm to maintain the same numerical

value after the change in the volt representation.^

We generally recommend that this procedure not be

used with presently available solid-state references

for two reasons. First, physical adjustment of the

output of solid-state standards usually causes the

voltage to continue to change slightly after any adjustment has been made because of settling of the adjustment potentiometer. It is therefore difficult to adjust the voltage to the exact desired value, and the

value must be monitored for unusual drift after the

adjustment has been made. Second, not all solidstate references have sufficient adjustment range to accommodate the change. For example, one model of a solid-state reference has an adjustment range of

only 5 ppm. Even if the specified adjustment range is sufficient to accommodate the change, the adjustment

potentiometer on a particular reference may unfortunately be at the top of its range and not provide sufficient adjustment. Therefore, we recommend that the procedure used for standard cells be followed for

solid-state references. Change only the assigned value

of the reference and do not physically adjust the actual voltage. As in sec. 3.2, the change in value is ac- complished by multiplying the January 1, 1990, numerical value of the reference voltage by the factor 0.999 990 736 to obtain the new corrected value. After the correction is made the value of the reference

voltage will be smaller. (See IMPORTANT on page 8

Note the example in sec. 2.3. The liter increased in size and it now

requires more liquid to fill the new 1-liter graduated cylinders.

1.0181216

1.0181214

e^ 1.0181212 -

Si ^ Z 1.0181210

>

1.0181208

(0o 1.0181206

>

1.0181204

1.0181202

upper control limit

o o ^ QP pO °o°

O O O O O QrQ (V Mean

X> 00 o" ^ o o°^^ CO

Lower control limit

11/1/89 12/1/89 1/1/90

Date

2/1/90

Fig. 2. Hypothetical control chart for a voltage reference prior to January 1, 1990.
1.0181220

= 1.0181200

(0>

75u

0)E

3

C

0)D>

(B

1.0181180

1.0181160

> 1.0181140

1.0181120 -

I

11/1/89 12/1/89

Date

1/1/90 2/1/90

Fig. 3. Hypothetical control chart for a voltage reference with the scale adjusted to accomodate

the entire change in the volt representation.

for a discussion on determining the January 1, 1990,

value of a reference standard.)

For example, given the January 1, 1990, numerical

value of a solid-state reference based on V(NBS-72),

multiply it by 0.999 990 736 to obtain the value for January 1, 1990, based on V(NIST-90). Thus,

10.000 054 3 V(NBS-72)

X 0.999 990 736 V(NIST-90)/V(NBS-72) = 9.999 961 7 V(NIST-90)

.

3.4 Updating Control Charts

Control charts of reference voltages (plots of refer- ence voltages vs time), should be used in every stan- dards laboratory as a means of determining whether

or not the reference voltages (and other process parameters) are in a state of statistical control [15]. Figure

2 (on page 9) shows a hypothical reference voltage

control chart with upper and lower control limits that one might have generated up to the end of 1989. The

change in the volt representation on January 1, 1990,

will introduce an unacceptably large step change in

later data plotted on the chart. In this example, the

mean value of the plotted points is 1.018120 91

V(NBS-72). Applying the correction for V(NIST-90)

will change the mean value to 1.018 111 48 V(NIST-90)

which will obviously cause newly plotted points to

be off the graph. If the range of the graph is increased

to encompass both the old and new points, the dis- tance between the upper and lower control limits will

be so small as to be indistinguishable from the points

as shown in Fig. 3. Therefore, in order to continue to be useful, the control chart will need to be modified.

Two alternative solutions to this problem are suggested below.

(1) To avoid the time consuming job of manually

replotting all of the data on control charts, one may

simply provide a second y-axis to be used for plotting data obtained after January 1, 1990. Figure 4

shows how this might be done. The additional y-axis

scale on the right is shifted by -9.264 ppm of the

mean value, or -9.43 |iV for this example. Thus the

lines drawn to represent the mean value and upper

and lower control limits continue uninterrupted. Do

not fail to note on the chart that the change is effective on

January 1, 1990.

(2) In many cases control charts are generated automatically by a computer. If the computer charting

program provides the capability of having a second

y-axis scale, the approach suggested in (1) above may

be used. However, with computer charting programs

it is often necessary to generate a completely new

control chart with the data taken before January 1, 1990, "corrected" to V(NIST-90). (Note: Do not alter permanant data records, see sec. 3.5.) The mean and

10
1.0181216

1.0181214

27 1.0181212

CO 1.0181210

>

"" 1.0181208-1

o

O)

f

1.0181206

>

1.0181204

1.0181202

*^ QCO cP °oO.

o %^-o—o-o-^U>cfi^—^

^ « CO oo ^ o o o do

Oo

CHANGE IN THE VOLT

REPRESENTATION ON

01/01/90; USE THE

SCALE AT RIGHT '

nn(^ no

° qO °o -- - ^%

11/1/89 12/1/89 1/1/90

Date

2/1/90

1.0181120

1.0181118 <

o

0) 1.0181116 »

1.0181114 ^

CO 1.0181112 ^

o

-1.0181110 "

1.0181108

Fig. 4. Hypothetical control chart for a voltage reference with an added scale on the right

to be used for all data after January 1, 1990.

1.0181122

1.0181120

ZZ 1.0181118

o

\^ 1.0181116

Z. 1.0181114-

I* 1.0181112

o

>

1.0181110 -

1.0181108

ALL EMFS SHOWN IN TERMS OF THE NEW

VOLT REPRESENTATION

o

-o-

°0($>

O ^ CEO pO °00_ ° " o o o o<P- <P o o ~o oo

Oo

%5^%r-^ Cb ^O __ = ^_

11/1/89 12/1/89 1/1/90

Date

2/1/90

Fig. 5. Hypothetical control chart for a voltage reference with the original scale modified to the new volt representation and the data before January 1, 1990, "corrected" to the new representation.

11
upper and lower control limits are also shifted by the same correction to provide a continuous record as shown in Fig. 5. Note that the plotted values before

January 1, 1990, are a construct; they are shifted only

for the purpose of examining the performance of a

particular voltage reference using the control chart. Do not use them as the calibrated values.

3.5 Do Not Correct Pre-1990 Data

Most standards laboratories maintain, or should

maintain, a historical record of the calibrations of

their standards and the intercomparison measure- ments made among their standards. On January 1, 1990, there will be a step change in the values of the

voltage references if the laboratory has properly adjusted their local volt representation to be consistent

with that of NIST. There may be a temptation to go

back and apply the change to all data prior to January

1, 1990, to make them consistent with later data. This

is incorrect; data taken prior to January 1, 1990, should

NOT be corrected to V(NIST-90) because V(NBS-72)

was in fact the legal unit at the time.

It may be the case that these historical records are maintained in a computerized data base. There are so many dangers associated with correcting the data in a computerized data base that we DO NOT recommend changing the original data. Some of the reasons follow:

• It is possible that some data files may be over- looked during the "correction" process and will

thus be incorrect.

• The "correction" process may be inadvertently applied twice to a particular data file. • The correction may be applied with the wrong

sign.

• The user or another user may forget that the data

is to be used only for control charts, and attempt to use the numbers as the calibrated reference val- ues.

In all these cases it is possible that, even if the

changes are carefully documented, an error in the

correction process may result in erroneous data that can never be properly corrected.

3.6 Effect of the International Temperatiure

Scale of 1990 on Standard Cells

On January 1, 1990, the International Practical Temperature Scale of 1968 (IPTS-68) will be superseded

by the International Temperature Scale of 1990

(ITS-90) [3, 16]. As a result of this change in scale,

temperatures will change about 0.006 to 0.008°C in

the range relevant to voltage standards (28°C to 37°C); see ref. 3 reprinted in Appendix 3. The change is analogous to the change in the volt and ohm representations, i.e., the temperature scale is shifting and

the "labels" will change but not the actual temperatures. This change may affect laboratories using standard

cells in an oil bath or in an air bath where the temperature

is measured using a calibrated platinum resistance ther- mometer or other temperature measuring device calibrated in terms of IPTS-68. Not affected are laboratories with

only solid-state references, or standard cell air baths

containing their own imbedded temperature monitor

such as a thermistor or a mercury-in-glass thermometer that is not removed and calibrated. Standards in the latter category define their own local temperature

scale and there is no need, or reason, to make any

adjustment to bring them into conformance with

ITS-90. Also not affected are those laboratories that do not "temperature correct" the values of their stan- dard cells (presumably only because of relatively low

accuracy requirements or excellent temperature bath

stability).

The introduction of ITS-90 will result in temperature values being shifted downward by about 7 mK

at 30°C. Thus if one were to maintain a constant-tem- perature oil bath at 30.000°C that does not physically

change temperature, after January 1, 1990, the "new"

temperature would be 29.993°C. If one follows tradi- tional procedures after the change and applies a temperature correction to the standard cells because the

cells were not at their expected or "nominal" temperature of 30°C, the application would cause the

emf's to be decreased by 0.4 \xV when in fact they did

not change. There are two possible ways of avoiding

this error.

(1) One may physically adjust the temperature of

the oil bath upward by 7 mK to bring it into

agreement with the new temperature scale. This

would produce a real decrease in the cell emfs of about 0.4 |iV. Control charts are handled in

exactly the same manner as for the volt repre- sentation change. It is also advisable to measure

the emf change of the cells in the bath (referenced to another group of cells whose temperature is not physically changed) to determine the exact change in emf, rather than relying on the

Wolff or International Temperature Formula.

(2) If the cells are always kept in the bath and are

I

12
not removed and sent to another laboratory for

calibration, the nominal temperature of the bath

may be simply adopted as 29.993°C. The nominal temperature of a bath is just a value chosen

to be considered the normal operating temperature of the bath, and need not be closely related

to any actual measured temperature. The nominal temperature is only a convenient numerical

reference. To see how this works, a discussion

of the temperature correction process follows.

When the cells are measured, their temperature

is also measured and a correction is applied if their temperature is different from the nominal

temperature. Later, when they are again measured, a second temperature correction is applied

for the difference between the current temperature and the nominal temperature. The important parameter is the difference between the two

temperature corrections; the nominal temperature drops out of the equation entirely.

4. Change in the U. S. Ohm Representation

The last change in the U. S. representation of the

ohm, ^(NBS-48), , occurred on January 1, 1948, when

the system of electrical units designated as "international" was superseded by units derived from the

fundamental units of length, mass, and time. These

have now evolved to become the International System of Units or SI. Since that time n(NBS-48), has

been based on the mean resistance of reference groups

of 1-Q resistors whose mean values were thought to be constant with time. Quantized Hall resistance

measurements made on a regular basis since August

of 1983 at NIST indicate that Q(NBS-48), , based on

the same particular reference group of five l-Q. resis- tors in use since 1972, has a drift rate of (-0.0529 ±

0.0040) ppm/year [14]. In calibration reports for re- sistance standards issued by NIST prior to January 1, 1990, the reported resistance values have not been

adjusted to eliminate the drift of 0(NBS-48),

.

4.1 Conversion Factor

As discussed earlier, on January 1, 1990,

a(NIST-90) will be based on the quanhim Hall effect in which a resistance is related to the fundamental

constants h/e^. The quantized Hall resistance, R^, is defined as the quotient of the Hall voltage U^^ of the

zth plateau to the current I in the Hall device and is given by

K„,„ = 25 812.807

a

(4.2)

RJi) = Uj,(i)// = R^/i (4.1)

where Ry_, the von Klitzing constant, is equal to h/e^

and i is an integer. The value of Q(NIST-90) will be

consistent with the internationally accepted conventional value of the von Klitzing constant, i.e..

exactly. This conventional value is believed to be

consistent with the true SI value to within a 0.005 D. assigned one-standard-deviation uncertainty, corre- sponding to a relative uncertainty of 0.2 ppm [1]. This change will decrease the numerical value of the

mean resistance of the NIST reference group by 1.69 ppm and eliminate the drift of the U. S. representation

of the ohm, as shown in Fig. 6. The conversion factor

for adjusting values of resistance standards in the

U.S. on January 1, 1990, is as follows:

1 a(NBS-48)o,/oj/,o = 0.999 998 31 Q(NIST-90). (4.3)

(Recall that Q(NBS-48), has a date subscript to indi- cate the time dependency of the U. S. representation

of the ohm before January 1, 1990.) In theory, the assigned values of all standard resis- tors based on NIST calibrations should be decreased by 1.69 ppm on January 1, 1990. The change in resis- tance of a standard from its last NIST calibration up

to January 1, 1990, should be included if its drift rate

is known. Also, the drift rates of standard resistors that are based on NIST calibrations should be decreased by 0.0529 ppm/year.

In practice, it is recommended that adjustments

only be made to the values of standard resistors whose

calibration uncertainties are roughly within ten times

the January 1, 1990, change in the U. S. representation

of the ohm. This means that only the values of stan- dard resistors based on Q(NIST-90) with reported

uncertainties of 20 ppm or less need to be adjusted.

These include standard resistors calibrated by NIST

in the decade resistance levels from 0.001 Q to 1 MQ.

13
o

a

E

p

0)u

c

S!

0)

E

a.

Q.

+0.3 -]

0.0

-0.3

-0.6

-0.9

-1.2

-1.5

-1.8

I

Q(NIST-90)

j quantum Hall effect

< no drift

I Q(NBS-48)^

five 1 -Q. reference resistors

drift = (-0.0529±0.0040) ppm/year

1 .69 ppm

n I 1 I I r 1984 1986 1988

Years

1 1 r 1990 1992

Fig. 6. Time dependence of the U.S. ohm representation.

Also, for standards in this resistance range, it is re- comn\ended that adjustments only be made to their

drift rates if these are 1 ppm/year or smaller in absolute magnitude. It is difficult to determine drift rates of resistors to better than 10%, and neglecting the

-0.0529 ppm/year drift of £l(NBS-48), prior to January 1, 1990, would result in errors of 5% or less for standard resistor drift rates with absolute magnitudes

of Ippm/year or larger.

4.2 Examples

The following examples are given to illustrate the

method of calculating and applying these changes.

(Note that the drift rate of a(NBS-48), may be rounded

to 0.05 ppm/year for these examples with negligible

effect.)

Example 1: A 1-Q standard resistor was calibrated

at NIST on 09/15/88 and reported to have a value of

0.999 998 91 Q(NBS-48)o5/j5/g8. From previous NIST

calibrations, its drift rate was calculated to be -0.10 ppm/year. Note that this drift rate includes the drift of Q(NBS-48),. What are its new value and drift rate on 01/01/90?

Solution: Calculate the change in resistance AR

due to drift from 09/15/88 to 01/01/90:

AR = drift rate x time interval = (-0.10 ppm/year) x (01/01/90 - 09/15/88) = (-0.10 ppm/year) x 1.29 years = -0.13 ppm

= -0.000 000 13 Q{NBS^8\,,^,,^,,,^^.

Add AR to the value for 09/15/88 to obtain the resis- tance R for 01/01/90 based on r2(NBS^8),

:

(

R(01/01/90) = 0.999 998 91 Q(NBS-48), = 0.999 998 91 Q(NBS-^8)

09/15/88 + AR

09/15/88

- 0.000 000 13 Q(NBS-48)

= 0.999 998 78 Q(NBS-48),

01/01/90-09/15/88

01/01/90"

Multiply by the conversion factor [eq.( 4.3)] to obtain

the resistance R for 01/01/90 based on Q(NIST-90);

for convenience in these Guidelines, this resistance

will be denoted as R^:

R, = 0.999 998 78 Q(NBS^8)oj/oi/,o x 0.999 998 31 Q(NIST-90)/a(NBS-48)(,j/oj/,o = 0.999 997 09 i^(NIST-90).

14
Calculate the new drift rate D: D = old drift rate + drift of Q(NBS-48), = -0.10 ppm/year - 0.05 ppm/year

= -0.15 ppm/year.

Therefore, on 01/01/90 the new value for this l-Q.

standard resistor is 0.999 997 09 Q(NIST-90) and its new drift rate is -0.15 ppm/year.

Example 2: A 10-kQ standard resistor was cali- brated at NIST on 03/15/89 and reported to have a value of 10 000.065 Q(NBS^8)

R„= 1000 072 Q(NBS^8)oj/oi,^

03/15/89' From previous

NIST calibrations, its drift rate was calculated to be

+0.20 ppm/year. What are its new value and drift rate on 01/01/90?

Solution:

AR = (+0.20 ppm/year) x 0.80 year = +0.16 ppm

= +0.0016 Q(NBS-48)„'01/01/90-03/15/89

R(01/01/90) = 10 000.065 Q(NBS^8),

+ 0.0016 Q(NBS-48), = 10 000.067 Q(NBS-48),

03/15/89

01/01/90-03/15/89

01/01/90

R, = 10 000.067 Q(NBS-48)oj/oj/,o X 0.999 998 31 Q(NlST-90)/i2(NBS^8), = 10 000.050 a(NIST-90)

D = +0.20 ppm/year - 0.05 ppm/year

= +0.15 ppm/year.

01/01/90

X 0.999 998 31 ii(NIST-90)/Q(NBS^8), = 1 000 070 a(NIST-90)

01/01/90

D = +2.0 ppm/year.

Therefore, on 01/01/90 the new value for this 1 MQ

standard resistor is 1 000 070 Q(NIST-90). Its drift rate remains at +2.0 ppm/year because the drift rate of Q(NBS-48), is negligible compared to the drift rate of this standard resistor.

43 Updating Predicted Values

and Control Charts

The usual method of determining whether resis- tance measurements are in a state of statistical control

is to examine the behavior of check standards. Check

standards are resistors, of quality equal to that of

reference resistors, which are treated as unknowns to monitor the operation of a resistance measurement

system. The measured value of a check standard, or any other standard, can be (1) compared to its pre- dicted value, or (2) plotted on a control chart with

upper and lower control limits to give evidence of the

quality of the measurements. The system is in statistical control when the measured values lie within a reasonable confidence interval. It is important when

updating predicted values or control charts of stan- dards to keep intact the original pre-1990 data (see

sec. 3.5 on correcting historical data).

Therefore, on 01/01/90 the new value for this 10-kQ

standard resistor is 10 000.050 Q(NIST-90) and its new

drift rate is +0.15 ppm/year.

Example 3: A 1-MQ standard resistor was cali- brated at NIST on 06/15/87 and reported to have a value of 1 000 067 a(NBS-48)og/j5/g7. Previous data

indicate it has a drift rate of +2.0 ppm/year. What are

its new value and drift rate on 01/01/90?

4.3.1 Predicted values

The resistances of well-aged standard resistors exhibit a linear functional relationship with time if the

resistors are not subjected to extreme temperature

changes or mechanical disturbances. The linear equation for this model to predict the value of a standard

at some future time can be written as:

Solution:

AR = (+2.0 ppm/year) x 2.54 years = 5 ppm

= 5 l2(NDS-48)Qj^pj^g(^Qg^j5^g7

R(01/01/90) = 1 000 067 i2(NBS-48), + 5 Q(NBS-48), = 1 000 072 Q(NBS^8),

06/15/87

01/01/90-06/15/87

01/01/90

R'{t) = R^ + {t-QD, (4.4)

where R'(t) is the predicted value of the standard at time t, Rg is the value for t^ = 1990.00 years, and D is the new drift rate. Examples of calculations of R^ and

D are given in sec. 4.2. The difference between the

measured and predicted values of the standard can be used to determine if the measurement system is in

statistical control, i.e..

15
TABLE II Correction Values est ppm to Convert to Q(NIST-90)

1985 1986 1987 1988 1989

IJan -1.426 -1.478 -1.531 -1.584 -1.637

IFeb -1.430 -1.483 -1.536 -1.589 -1.642

IMar -1.434 -1.487 -1.540 -1.593 -1.646

lApr -1.439 -1.491 -1.544 -1.597 -1.650

1 May -1.443 -1.496 -1.549 -1.602 -1.654

IJun -1.447 -1.500 -1.553 -1.606 -1.659

IJul -1.452 -1.505 -1.558 -1.610 -1.663 lAug -1.456 -1.509 -1.562 -1.615 -1.668

ISep -1.461 -1.514 -1.567 -1.619 -1.672

lOct -1.465 -1.518 -1.571 -1.624 -1.677

INov -1.470 -1.522 -1.575 -1.628 -1.681 IDec -1.474 -1.527 -1.580 -1.633 -1.686

-CL < m)-R'{t) < +CL, (4.5)

where R{t) is the measured value of the standard and

CL is the confidence level limit. This is probably the easiest method to determine

the control state of the measurement process, especially if the data analysis is being done by a computer. These equations can be incorporated into a computer program that indicates an error condition

or flag whenever the difference between the measured and predicted values of the standard exceed

the confidence level limits.

4.3.2 Control charts

As a result of implementing the new representation

of the ohm, Q(NIST-90), the control charts of stan- dards after January 1, 1990, will display significant

changes both in the measured values and subsequent

drift rates. After January 1, 1990, it will be difficult to determine graphically if the measurements are in a

state of statistical control until a sufficient data base is collected for the standard. Therefore, it will be neces- sary to correct the pre-1990 data on the standard for the change and drift of Q(NBS-48), prior to January 1, 1990. The correction c to the pre-1990 data will be a

linear function with time and can be expressed in

ppm as:

c = -1 .69 ppm - (0.0529 ppm/year)(f -

1^ ), (4.6)

where t^ = 1990.00 years, and t is the measurement

date expressed in years and is < 1990.(X). For the

convenience of the reader. Table II gives the correction

c for monthly intervals starting from 1985.

A model of a control chart for monitoring the val- ue of a 1-Q. standard after January 1, 1990, is shown

in Fig. 7. The filled-circle symbols represent measured values of the standard. The open-circle symbols

represent the pre-1990 data that was adjusted using

the corrections listed in Table II. As examples, two of

these corrections are printed on the chart, namely,

-1.64 ppm and -1.69 ppm for January 1, 1989, and

January 1, 1990, respectively. The corrected data

(open-circle symbols) are used to determine a new

trend line for the continuation of the control chart

after January 1, 1990. This can be done graphically, or more precisely by the method of least squares for linear regression. The upper and lower limit lines

about the trend line correspond to confidence level limits of ±0.10 ppm. Note that the drift rate of the standard has changed as a result of eliminating the drift of Q(NBS-48)^. Before January 1, 1990, the drift rate of

the standard was +0.067 ppm/year; after January 1, 1990, its drift rate is +0.014 ppm/year.

The range of corrections for the control chart in

Fig. 7 was expanded to include the -1.69 ppm shift on

16
E

a.

Q.

c

co

a>o

o

2.00

1.50

--

1.00

0.50 --

0.00

-0.50

-1.00

600000000000000000000000 •••••<

1988 1989 1990

Years

1991 1992

Fig. 7. Hypothetical low resolution control chart of a 1-Q standard.

1.500

E 1.375

Q.

O.c

coB 1.250-

o

0)o

o

1.125 --

1.000

1988

D(NBS-48), il(NIST-90)

-0.190

-0.315

-- -0.440

-0.565

o

o

o

o

5" a

•a3

-0.690

1992

Fig. 8. Hypothetical high resolution control chart of a l-Q standard. The added

scale on the right is used for all data after January 1, 1990.

17
January 1, 1990. Consequentiy, it is difficult to see the

variability of the measurements and the apparent

change in the drift of the standard. The resolution of

the control chart can be increased by creating one

with two scales for the y-axis. The y-axis scale on the

right is offset by -1 .69 ppm to force the trend line to be

continuous on January 1, 1990. This is shown in

Fig. 8. Note that the scale on the left refers to measurements made before January 1, 1990, while the scale on the right refers to measurements made on and

after January 1, 1990. With this control chart it is easier to determine the quality of the measurement

process.

4.4 Effect of the International Temperatiu"e

Scale of 1990 on Standard Resistors

The region of temperature of critical importance to resistance measurements is from 20°C to 30°C. Resistors are usually maintained in a laboratory environment at 23°C or in an oil bath at 25°C, and the temperature coefficients of resistors are usually determined over the temperature range from 20°C to 30°C.

On January 1, 1990, the International Temperature

Scale of 1990 (ITS-90) will supersede the International

Practical Temperature Scale of 1968 (IPTS-^8). ITS-90

is not expected to be finalized until the latter part of

1989. However, for most applications to resistance

measurements, preliminary analysis of available temperature data indicates that ITS-90 (over the temperature range from 20°C to 30°C) can be approximated as a linear shift in temperature, M,

ho =^68 + ^^ (4.7)

formance. Laboratories should consult Guidelines for Realizing the International Temperature Scale of 1990

(ITS-90) [16].

(2) Resistance Changes and Temperature

Coefficients of Resistors

The resistance-temperature curve for resistors over

the interval 20°C to 30°C can be represented by

R(0 = R'[l+a(f-g + p(f-f^)2], (4.8)

where R{t) is the resistance at temperature t, R' is the

resistance at a reference temperature t^,ais the slope

of the curve at t^, and P determines the curvature at any temperature. The reference temperature t^ is usually 25°C for oil-type resistors and 23°C for air- type resistors. To indicate which temperature scale is in use, eq.

(4.8) can be written as K(U = ^'68^1 + 0C,8(^68 -V + K^teS - K^^ <4.9) RitJ = R\,[\ + a,,it,^ -t^) + P,o(t,o

-

1^ n (4.10)

where the subscripts 68 and 90 denote that the temperature and coefficients are based on IPTS-68 and

ITS-90, respectively. The value of t^ is the same for both equations. Using eq. (4.7) along with eqs. (4.9) and (4.10), one can derive expressions relating the coefficients based on the different temperature scales. Neglecting negligible terms, these equations are:

= t,-(6±l)mK,

«90 = 0C68-2M^^)'

K - P68-

(4.11)

(4.12)

(4.13)

where tg^ and t^ are the Celsius temperatures defined

by ITS-90 and IPTS-68, respectively.

ITS-90 will affect resistance measurements primarily in three areas:

The change in resistance at the reference temperature, AR(f^), resulring from the implementation of

ITS-90 can be calculated from eq. (4.11) and expressed

as

(1) Oil Bath Temperatures mt) = R\,- R\,^R'[-ajAt) + ^jAtn (4.14)

On January 1, 1990, the NIST resistance laboratory

will physically adjust the temperature of its 25.000°C

oil baths as based on IPTS-68 so that they will be- come 25.000°C oil baths as based on ITS-90. (That is, the temperature of the baths will be physically in- creased by about 6 mK.) To remain consistent with

NIST, it is imperative that other laboratories also conform

to ITS-90. The following analysis assumes such conSince At is small, the P term in this equation can be

neglected. The maximum change in resistance for

resistors with a's within the limits ±10 ppm/K would

be ±0.06 ppm. This may be significant at the l-Q.

level; however, for most resistance measurements this change is negligible. In theory the values of all a's of standard resistors

should be adjusted on January 1, 1990, using eq. (4.12);

18
however, in practice the magnitude of this adjustment is negligible for most resistors. Since the values

of P are approximately -0.5 ppm/K^ for typical manganin-type resistors, a^^ for this type of resistor dif- fers from ttgg by less than 0.01 ppm/K in absolute

value. The effect is less for Evanohm-type resistors

since their values of P

are approximately -0.05 ppm/K^. The P coefficients do not change value, as shown in eq. (4.13). As an example to illustrate the changes resulting

from implementing ITS-90, consider example 1 from

sec. 4.2 and assume that the 1-Q resistor has an a^g of

+1.67 ppm/K and a p^^ of -0.50 ppm/K^. What are its change in resistance and its new temperature coeffi- cients of resistance on January 1, 1990?

Solution: Calculate the change in resistance AR(f^ ). From eq. (4.14), neglecting p^^

,

AR(f^) = J?y-a^(AO]

« 0.999 997 09 Q(NIST-90) x (-0.000 001 67/K)

X (-0.006 K)

- 0.000 000 01 a(NIST-90).

Its new value R which includes the change resulting

from the effect of ITS-90 is: R '^R^ + ARit^

= 0.999 997 09 Q(NIST-90)

+ 0.000 000 01 Q(NIST-90)

=0.999 997 10 a(NIST-90).

Calculate the new temperature coefficients of resis- tance:

0,0 = a^ - 2pJAf)

= (0.000 001 67/K) - [2 x (-0.000 000 SO/K^)]

X (-0.006 K)

= 0.000 001 66/K

= +1.66 ppm/K

p,o«p,«

= -0.50ppm/K2.

Therefore on 01/01/90 the new value for this 1-Q

standard resistor is 0.999 997 10 Q(NIST-90) and its 0,0 is +1.66 ppm/K, and its P,g is -0.50 ppm/K^. Note

that the changes that result from implementing ITS-90

are small and are only significant for resistance measurements at the 0.01 ppm level of accuracy.

(3) Control Charts

The implementation of ITS-90 will affect the trend

lines of control charts in addition to the alterations

discussed in sec. 4.3.2. The effect is only a shift of the

trend line and for most resistance measurements is negligible. However, this shift, ARit^ ) may be signifi- cant at the 1-Q level and may be calculated from eq.

(4.14). It is +0.01 ppm in the previous example.

4.5 Summary

To conform to the new representation of the ohm,

Q(NIST-90), and ITS-90 (as it affects resistance measurements), it is recommended that on January 1, 1990,

the following steps be taken:

1. Calculate R^, the resistance for 01/01/90 based on

Q(NIST-90) (see examples in sec. 4.2) for resistors

with uncertainties of 20 ppm or less.

2. Calculate D, the new drift rate after 01/01/90 (see

examples in sec. 4.2) for resistors with drift rates with absolute magnitudes of < 1 ppm/year.

3. The temperatures of oil baths operating at 25.000°C

defined by IPTS-68 must be increased by 6 mK to convert to ITS-90.

4. In general, there is no need to calculate new a coefficients for resistors. The change in the absolute

magnitude of a for most resistors would be < 0.01 ppm/K. Also, the P coefficients do not change.

5. Calculate AR{t^ ) (defined in eq. 4.14), the change in

resistance resulting from the implementation of

ITS-90, for Thomas-type (or equivalent) l-Q resis- tors with a's having absolute magnitudes > 1.7 ppm/K. This change is negligible for most other

resistance measurements.

6. The new value of resistance, R, for Thomas-type

(or equivalent) l-Q. resistors is the algebraic sum

of items 1 and 5. That is, i^ == Rq + ^(^r^-

7. Update procedure for calculating predicted values

of check standards as outlined in sec. 4.3.1.

8. Update procedure for monitoring resistance measurements using control charts as outlined in sec.

4.3.2. Most laboratories will be consistent with

Q(NIST-90) if the above recommended procedural

steps are followed. However, there may be some

critical resistance measurements where the effect of

ITS-90 is significant and steps 4, 5, and 6 would have

to be investigated in greater detail.

19
5. The Reporting of Calibration Results by NIST

The CCE believes that (i) the appearance of creat- ing a new unit system outside of the SI must be

avoided; (ii) the new volt and ohm representations

based on the Josephson and quantum Hall effects will be completely satisfactory for the great majority

of applications (i.e., it will rarely be necessary to distinguish between the new representations and the SI

units); and (iii) any differences among the volt and

ohm representations of different laboratories will be

negligible from the point of view of the great majority of users (i.e., it will rarely be necessary to distinguish between the representations of different laboratories). Therefore, the CCE and CIPM have recom- mended that national standards laboratories avoid

the use of subscripts or other distinguishing symbols

of any sort on either unit symbols (i.e., V, Q.) or quantity symbols (i.e., E, R), when reporting the results of

calibrations carried out in terms of the new volt and

ohm representations. Examples of such subscripts are those denoting particular laboratories or dates

such as Vjji^r, V^^, E^^, or E^^. The CCE's solution to the reporting problem, which was affirmed by the CIPM and which all national

standards laboratories are requested to use, is indicated in the following variation of the example given

by the CCE (the treatment of resistance measurements

is strictly analogous) [1, 3, 6]: The emf E of an unknown standard cell calibrated

in terms of a representation of the volt based on the

Josephson effect and the conventional value of the

Josephson constant K. ,p, may be rigorously expressed

in terms of the SI volt V as (to be specific):

E = (1.018 123 45) V±e, (5.1)

where e represents the total uncertainty (in volts) and

is composed of the following two components: AE,

the combined uncertainty associated with the calibration itself and with the realization of the Josephson

effect volt representation at the particular standards

laboratory performing the calibration; and AA, the

uncertainty with which the ratio K,^/K, is known

(i.e., it is assumed that K^^^/K, = 1 ± A/4). According

to the CIPM, AA may be taken as 0.4 ppm (lo).

Since, by international agreement, AA is common

to all laboratories and is not relevant for traceability to national standards, the two uncertainties AE and

AA need not be formally combined to obtain the total uncertainty but may be separately indicated. Hence,

the measured emf E may be expressed as

E = (1.018 123 45) V±AE (5.2)

for all practical purposes of precision electrical metrology and trade, with AA appearing separately on

the calibration certificate when the precision of the

calibration warrants it. If, for example, AE/E is significantly greater than 0.4 ppm, AA may be omitted

with negligible effect. In fact, the CCE approach is not too dissimilar to NIST practice prior to January 1, 1990, when calibration results were reported in terms of the 'U.S. legal

volt' as derived from the Josephson effect, but with

the uncertainty corresponding to AA always omitted.

In response to the CCE recomendation, starting January 1, 1990, NIST will include with all calibration

reports an information sheet discussing the existence

of the additional uncertainty component AA and giving its value. An example of the wording that will be

used on a NIST Report of Calibration for a standard

cell enclosure, and which in part is a variation of the

wording used in an example developed by the CCE,

is given in Fig. 9. Figure 10 is an excerpt from an

information sheet to be included with NIST reports of

calibration of saturated standard cells and solid-state

voltage standards. The information sheet to acom- pany NIST reports of calibration of high precision

resistance standards will contain a similar paragraph.

However, as previously indicated, an ideal representation of the ohm based on the quantum Hall effect and Rj,_5(, is expected to be consistent with the SI ohm

to within an assigned relative one-standard-deviation uncertainty of 0.2 ppm (0.2 \\£l for a resistance of

1 fl).

20
\

UNITED STATES DEPARTMENT Or COIVIIVIERCE

National Institute of Btandarda and Technology

[formerly National Bureau of Standards]

Gacheraburg. Maryland SOBS9

REPORT OF CALIBRATION

DC Voltage Standard

Description of Standard

Standard Cell Enclosure

Model S/N

Containing 4 Saturated Standard Cells

Submitted By:

This standard cell enclosure was received August 30,

normal operating teitperature.

1990, under power and at its

The values in the table below are based on the results of daily measurements of

the differences between the emf's of the cells in this standard and those of NIST

working standards calibrated in terms of the Josephson effect using the new con- ventional value of the Josephson constant internationally adopted for use starting

on January 1, 1990, namely, K^gj^ = 483 597.9 GHz/V exactly.* The measurements were

made in the period from Novenijer 2, 1990 to November 17, 1990.

POSITION EMF UNCERTAINTY EMF UNCERTAINTY

NUMBER (volts, V) (microvolts r^V) (volts, V) (microvolts , |iV)

1 1.0181770 0.27 1.0181776 0.27

2 1.0181787 0.27 1.0181794 0.27

3 1.0181775 0.27 1.0181781 0.27

4 1.0181775 0.27 1.0181781 0.27

The electromotive forces on the left above were corrected to nominal temperature

(30.0 degrees Celsius) using the International Teirperature Formula proposed by F. A. Wolff. The electromotive forces on the right are at the mean operating teirperature during the test (29.9887 degrees C) as determined by use of a temperature

deviation measuring device mounted in the enclosure.

The above uncertainties include coirponents for random fluctuations in the cell

under test and in NIST equipment and standards, for a systematic error of 0.076

ppm in the measurements of NIST working standards in terms of the Josephson effect, and for the systematic error in transfer due to the finite resolution of the

apparatus used to determine the tertperature of the cells under test. In the case

of standard cells tested in NIST oil baths, the latter uncertainty is replaced by

the emf equivalent of 0.005 degrees Celsius which is the uncertainty of the

*For additional information, see NIST Technical Note 1263, "Guidelines for Inplementing the new Representations of the Volt and Ohm Effective January 1, 1990," by

N. B. Belecki, R. F. Dziuba, B. F. Field, and B. N. Taylor (June 1989)

Test No.

Fig. 9a. Example of a NIST calibration report for saturated standard cells, page 1.

21
standard Cell Enclosure

Model S/N

tenperature measurements in this laboratory. The random error component is

coirputed from the standard deviation of the mean emf and is at the three sigma

level

.

These uncertainty figures contain no allowance for the effects of transportation

upon this standard. The minimum uncertainty due to such effects under very carefully controlled transport conditions has been found to be 0.42 ppm (3 sigma)

.

Any valid uncertainty statement applying to the above values when the standard

has been moved from the NIST Volt Facility must contain such a component of

error. If data from which to estimate the transport error are not available, one

part per million is not an unreasonable value. Also not included in the above

uncertainties is an allowance for long term drift of the values of the outputs of

this standard. This must be determined from historical data on a case by case

basis.

A summary and analysis of the data upon which the above values are based is

appended. A coitplete explanation of the uncertainty statements given above as well as additional information regarding NIST calibration of such voltage standards is also included.

For The Director,

National Institute of Standards and Technology

Norman B. Belecki, Group Leader

Electricity Division

Test No

Date: November 23, 1990

Fig. 9b. Example of a NIST calibration report for saturated standard cells, page 2.

22
As of January 1, 1990, the U.S. reference standard of voltage has been based on eq. (1) taking for the Josephson constant Xj ,„ = 483 597.9 GHz/V exactly. This value is that adopted by international agree- ment for implementation starting on January 1, 1990, by all national standards laboratories that base their national representation of the volt on the Josephson effect. Since all such laboratories are expected to use

the same conventional value of the Josephson constant while prior to this date they did not, the

significant differences which previously existed among the values of some national representations of the

volt should no longer exist [4]. Moreover, the national standards laboratories of those coimtries that do

not use the Josephson effect for this purpose are requested to maintain their own national representation

of the volt to be consistent with the above conventional value of the Josephson constant, for example,

through periodic comparisons with a laboratory that does use the Josephson effect. An ideal representation of the volt based on the Josephson effect and K.^^ is expected to be consistent with the volt as defined

in the SI to within an assigned relative one-standard-deviation uncertainty of 0.4 ppm (0.41 |i.V for an emf

of 1.018 V). Because this uncertainty is the same for all national standards laboratories and is not relevant

for traceability to national standards, it is not included in the uncertainties given in NIST Reports of

Calibration. However, its existence must be taken into account when the utmost consistency between

electrical and nonelectrical measurements of the same physical quantity is required. (Examples are the

electrical and mechanical measurement of power, and the electrical and thermal measurement of energy.)

Fig. 10. Excerpt from an information sheet to be included with NIST reports of calibration on saturated standard cells and solid-state voltage standards.

6. Adjustment of Instrumentation

It is important to follow manufacturers' instructions

when making any adjustments to instrumentation. It must always be remembered that recommendations

in this section are general guidelines only, not specific procedures. Following procedures and instructions specific to your instruments is the only sure way to avoid errors and problems.

6.1 Voltage Instrumentation

Calibrators and precision sources - Calibrators

should be adjusted using a standard calibrated to be

consistent with V(NIST-90). A standard in this context means any device with sufficient stability, resolution, and absense of noise to transfer the representation with an uncertainty smaller than the calibrator's smallest uncertainty by a factor of three or more. The adjustment to be made is generally that of the

internal reference of the calibrator and such an adjustment affects all selectable output levels proportionally. For that reason it may be sufficient to perform the adjustment at one voltage level, say, ten- volts nominal. The adjustment would have the result of increasing the positive voltage output at any fixed

setting by 9.264 ppm of prior value if the calibrator were perfect. However, no calibrator is perfect and

one can safely assume that the change in output would

differ from +9.264 ppm by some amount due to a combination of calibration errors and instrument drift since the last calibration. For purposes of verifying

the adjustment, the manufacturer's accuracy and drift specifications or your own past calibration data on

the calibrator may be used to estimate the extent of

the difference likely to be obtained.

The adjustment may be checked by measuring one

or two output levels on each voltage range with a

digital voltmeter, of 6 1/2-digit resolution or better,

immediately before and after the adjustment is made.

The increase of voltage in proportional parts of each

of the readings should be the same, taking into ac- count the linearity specification of the calibrator.

For example, assume this was done, and the re- sults appearing in Table III were obtained.

23
TABLE III Calibrator Outputs Before and After Adjustment

Range Before Data After Data A (ppm)

10 Volt 10.00002 10.00010 8.0 -10.00005 -10.00012 7.0 100 V 100.0003 100.0011 8.0 -100.007 -100.0014 7.0 1000 V 1000.005 1000.014 9.0 -1000.002 -1000.010 8.0

The proportional change in ppm was calculated

using the equation:

After - Before

A =

Before

xlO^

Two observations can be made from these data.

The first is that the differences A are consistent within

the resolution of the meter used to obtain them. In- consistenties would have indicated a possible procedural error in making the adjustment. The second is the differences have the correct sign and are of about

the right magnitude. This indicates that most likely

the adjustment has been made properly. If the differ- ences were very far from 9 ppm, a recheck using an

independent standard would be advisable. Note also

that it is possible (but unlikely) that the effect of drift in the calibrator since its last calibration and an error

in making the adjustment could cancel one another,

leaving the impression that the adjustment was correctly performed.

Digital voltmeters and multimeters - The best

way to perform the adjustment on a digital meter is to use a calibrator of appropriate accuracy to adjust its internal reference based on a nominal ten-volt input

to the meter, thus following the same approach used

in the case of calibrators. This will have the effect of

reducing the reading obtained when measuring a fixed

voltage before and after the adjustment. Accordingly, to ensure that the adjustment was properly

made, one may simply measure one or two voltages on each range before and after the adjustment.

6.2 Resistance Instrumentation

Measurements of resistance at accuracy levels high

enough to be affected by the change in the ohm repre- sentation are made only by a few of the highest resolution digital multimeters and perhaps one or two

calibrator models. In the case of a digital multimeter,

the measurement is usually made by measuring the

voltage drop across the unknown resistance caused

by a constant current produced by a source within

the meter. In adjusting the meter, the current is physically adjusted or corrections stored while reading a sequence of standard resistors, typically one per range. Unlike the case of voltage adjustments, the 1.69 ppm

change will be difficult to see as it is small relative to the resolution of even the best meters.

6.3 General Considerations Regarding

Adjusting Instruments

6.3.1 Adjust instruments without using

standards at yoiu* own risk

It is possible to bring instruments into compliance

with the new representations in a technically-sound

way without recourse to standards. However, such

procedures are not recommended by the Ad Hoc

Committee because of logistics problems they may

create. As an example, consider the adjustment of a DVM. The adjustment can be done by connecting it to a stable ten-volt source, noting the DVM reading,

computing what the reading should be after the

change, and adjusting the reference circuit of the DVM

to obtain the new reading. Even though this would

permit the adjustment of the DVM in situ, without

involving standards, it is not a preferred procedure be- cause of the risks involved. Unless you are careful to mark the DVM somehow, e.g., with the logo described

in sec. 2.2.4, the danger exists that it could be adjusted more than once and therefore have an offset large

enough to put it outside of its specifications. It should

also be recognized that the above procedure is illus- trative only. Other pitfalls possibly encountered in

this approach include:

• High likelihood of making mathematical errors in

calculating the adjusted reading.

• Overlooking the existence of a second internal

reference in, for example, dual-slope integrating

voltmeters.

• Failing to compensate for zero offsets.

• Failing to understand that any error existing in the instrument prior to the adjustment is propagated unaltered through the adjustment under

this procedure, i.e., the procedure has none of the

aspects of a calibration.

24
If you insist on adjusting instruments without using

standards, it is imperative that the manufacturers' adjustment procedures be taken into account.

63.2 Where should the adjustments be

made?

Since complete recalibrations are not necessary to bring meters and sources into compliance with the

changes (see sec. 22.1), it is possibile to make the adjustments at the user's site. This is ideal for it would

enable the change at the user level to be made quickly,

allowing compatibility of measurements in a large

production or quality-assurance system to be maintained readily. It also enhances the value of the meas- urement results from instruments so adjusted because

the adjustment is made under use conditions, thus

eliminating the effects of differences between the en- vironments of the calibration laboratory and the user's

site. In many cases this difference can be extreme; in

the calibration laboratory, the instrument is calibrated on a bench top, but when in use it will likely be in a rack with other heat-generating equipment.

Most modern voltmeters and calibrators whose

accuracies are high enough that they will be affected

by these changes are "smart" instruments. That is, they contain microprocessors for control and operational purposes. Most also contain non-volatile

memory used to store calibration corrections. The

references of these can be adjusted by using a procedure based on the idea of measuring a source and

then "telling" the instrument its value. The meter

then computes and stores a new value for its own

reference. The simplicity of this makes in situ adjustment feasible. However, the adjustment should be

done only with a source calibrated to sufficient accu- racy and a logo or label should be used to indicate

that an adjustment has been made.

Digital instruments and precision sources of older

design — i.e., those not using microprocessors for operation and control, will require manual adjustment of reference outputs. Because of this, the feasibility of making the required adjustments at the user's

site is diminished. For such cases, the decision

whether to pull the instrument out of service merely

for adjustment or to perform a complete calibration

depends on the circumstances. Since adjusting refer- ences of instruments is not a particularly time-consuming task, it may be worthwhile to dedicate a technician and work station to do nothing but adjust test equipment voltage and resistance references. This

will guarantee that instruments are not taken out of

service for an inordinate time and that the calibration

laboratory work schedule is not artificially jjerturbed

by the changes.

There will undoubtedly be digital instruments, and

perhaps even standards, for which the range of adjustment is insufficient to permit the entire change to be made. In such cases, circuit modifications may be

necessary. The manufacturer of the instrument should

be consulted prior to making such alterations. In the case of standards, good practice suggests that no

physical adjustments be made to them. Rather, a record of their values, both recent and historical,

should be maintained and used to determine a "best" current value. That is, standards should not be physically adjusted to their new nominal values; any

changes should be in record keeping only.

Many instruments affected by these changes are embedded in computer-controlled test or measure- ment systems. Where such instruments serve as the

reference standards for the system, the adjustments

should be made as soon as practical if the system is required to make measurements with uncertainties of

100 ppm or smaller. The system itself should be self- calibrated immediately after the adjustment is made

to the reference standard. Data from the self-calibra- tion should be compared with data from previous

self-calibrations to ascertain that the adjustments have

been properly made. If there is any question, a separate calibrated standard should be used to check the

system as a whole.

In principle, adjustments for the changes could be

made in the system software. This is not good practice for cases where a precision digital multimeter or programmable source or calibrator is used as a stan- dard for the system. In addition to the possibility of

creating blunders in the system software, this approach creates a problem if the instruments involved

have to be replaced for maintenance or other reasons.

It is far better to make the adjustments to the instruments themselves. In cases where a precision analogto-digital converter (ADC) or digital-to-analog converter (DAC) is used as the primary reference for a

system, corrections are generally stored in the system

software. These corrections must be sought out and

altered. All adjustments should be thoroughly docimiented.

If shifts in test results appear, or are likely to appear,

because of the adjustments, notification should be

given to those affected so that they can determine if their process has gone out of control.

25
7. Other Quantities

7.1 Direct Current

Since A = V/Q, where A is the SI ampere, the U.S. representation of the ampere will increase by +7.57

ppm. The new representation will be consistent with

the SI ampere to within 0.45 ppm (la). However, no

standards of direct current per se are kept at NIST or

at any other national standards laboratory. Known

currents for calibrating current sources, ammeters, or

the current function of digital multimeters are established using resistance and voltage standards, the

treatment of which is covered in sees. 3 and 4 above.

According to the criteria in sec. 2.1, only meters and

sources capable of measuring or producing currents

at the 80 ppm level of accuracy or better need be considered for adjustment.

7.2 Alternating Voltage and Current

All measurements of ac voltage and current, in- cluding calibrations of ac calibrators and the ac function of digital multimeters, are affected by the changes

in the volt and ohm representations since they are based on dc standards. The magnitude of the change

is the same as that for a dc measurement of the same

quantity.

Calibrations of ac instrumentation of the highest

accuracy are based on the use of dc standards and

thermal transfer standards (thermal voltage and cur- rent converters). The latter have a calibrated and

nearly flat frequency response. They are used to compare rms ac signal levels with nearly equal dc

levels produced or measured by dc standards. The

uncertainty at any calibration point is a function of

the uncertainty of the dc standard and that of the ac- dc difference of the transfer standard at that frequency

and voltage or current level. At frequencies above 20

kHz, the uncertainty associated with the transfer stan- dard may dominate, increasing from a possible low

of a few ppm in the audio frequency range to hundreds of ppm at frequencies above 1(X) kHz [17]. The ac-dc differences of thermal converters are not

affected by the change in the unit representations.

They depend on the materials and geometry of the

thermal converters and are determined ultimately on

theoretical grounds at NIST and other national stan- dards laboratories.

AC calibrators make the routine calibration of the ac functions of large numbers of digital multimeters

feasible. Such calibrators are precision adjustable

sources, usually programmable, which supply very

pure sine waves whose amplitude and frequency may

be selected with a precision of one ppm or better. They are calibrated using thermal voltage converters and a dc voltage calibrator. The uncertainty of the

calibration at each point is limited by the stability of

the source itself, the dc calibrator, and the uncertainties of the thermal converters. The very same considerations apply to the adjustment of an ac calibrator as

to a dc calibrator. The adjustment need only be made

at one point since the level of the internal reference is what will be adjusted; see sec. 6.1. Digital multimeters generally make use of an rms- responding thermal converter, a log-antilog converter, or sampling techniques to make ac voltage measurements. In the first two cases, the converter produces

a dc voltage which is measured by the dc voltmeter.

In the third, the waveform is sampled repeatedly, dc

measurements are made of the samples, and the rms

value of the waveform is calculated by a microprocessor in the meter and displayed. The calibration

technique for each case is the same, namely, to supply

a known voltage at a known frequency from an ac

calibrator and thereby determine the error of the meter. (Present meters simply have no facility for independent analysis of the converter circuit.) Since a conversion to dc is made and a dc voltage measured

to determine the ac voltage, the adjustment of the dc

function — by adjusting its reference — should be

sufficient for ac as well. However, the manufacturer's

instructions should be checked to be sure. As in the case of a dc meter, the measurement of a fixed voltage

just after the adjustment is made should result in a lower value than that from a measurement of the same voltage immediately prior to the adjustment.

The same accuracy levels apply to ac quantities as

to dc quantities for determining if adjustments for the

new representations are to be made. The only differ- ences in the two situations are that for ac the accuracy

depends on the frequency as well as the amplitude,

and that the accuracy levels are considerably lower

because of the performance limitations of the con26
verters or sampling schemes used. Therefore, far fewer ac instruments will be affected than dc instruments.

73 Capacitance and Inductance

A calculable cross-capacitor is used at NIST to re- alize the SI farad. The results of such realizations in

1961 and 1974 were used to determine the mean

value of the group of 10-picofarad, fused-silica ca- pacitance standards which maintain the U.S. farad

representation. All calibrations of capacitance stan- dards at NIST are ultimately based on this mean value.

The most recent realizations (1988) of the SI farad

at NIST indicate that the current NIST farad repre- sentation exceeds the SI farad by approximately 0.14

ppm [18]. (This is a preliminary figure as further

measurements will be carried out during 1989.) Thus, on January 1, 1990, the U.S. farad representation will

be reduced by approximately 0.14 ppm.*

In keeping with the previously-expressed rule delineating the accuracy levels over which the change

has impact, no capacitors whose calibration uncertainty exceeds 2 ppm should be affected. This limits

the impact to the calibration of fused-silica dielectric standard capacitors for which the range of calibration

uncertainties falls between 0.5 and 1.5 ppm. For these,

the adjustment must be made.

Note that, unlike V(NBS-72) and n(NBS-48),, the

U.S. farad representation is too large and will be

decreased. This means that assigned values of standard

capacitors must be increased by 0.14 ppm of nominal value

if the last calibrated value is used as the current value. If the calibration history of the capacitor is used to produce a curve from which daily values of the standard

are projected, the curve must be translated up by 0.14

ppm or the scale of the y-axis changed appropriately

(see sees. 3.4 and 4.3). The U.S. representation of the henry is realized via a Maxwell-Wien bridge as the product of the values

of two resistors and a capacitor. In principle, therefore, it should change by about (1.69 + 1.69 - 0.14)

ppm or 3.24 ppm. However, because the lowest uncertainty currently given for NIST calibrations of stanThe new NIST repiresentation of the farad will be very close to the SI farad. The uncertainty of a realization is of the order of ±0.015 ppn\ (la). Because of the intrinsic coherence of the SI system, the NIST representation

of the farad will be consistent with those of the volt and ohm to well within

the latter's assigned uncertainties.

dard inductors is 200 ppm, a change of 3.24 ppm will

have no practical effect on values of inductance. Therefore no adjustment is necessary.

7.4 Power and Energy

The SI unit of power, the watt, is related to the volt

and ohm by the equation:

W = Y^/Q.

The U.S. electrical representation of the watt therefore will increase by 16.84 ppm and will be consistent

with the SI watt to within 0.83 ppm (la). The U.S.

electrical representation of the unit of energy, the

watthour, will also increase by the same fraction.

A calibration of a wattmeter produces a correction or corrections to be applied to its readings to obtain

the correct value. To comply with the change in the

watt representation, the correction must be made more

negative by 16.84 ppm. Such corrections should be

made as soon after January 1, 1990, as is feasible for

all wattmeters used at the 85 ppm level of accuracy or

better. No adjustment is necessary to the corrections

of meters with accuracies of 170 ppm or greater. Meters whose accuracies are between 85 and 170 ppm

fall into a "gray" area where a decision to adjust must

be based on the criticality of the application, the time

remaining before the next scheduled recalibration,

and other such factors [19]. Watthour meter calibration results are expressed

in terms of "percent registration." For example, if a watthour meter spins too fast (indicates an erroneously

higher level of energy) by 0.1%, the reported value is 100.1% registration. In order to comply with the

change in the watt representation, the percent registration value for a given meter should be decreased

by 16.84 ppm (0.001 684%). The guidelines given in

the paragraph above should be used to determine the cases for which an adjustment is appropriate [20].

It is expected that only the most accurate wattmeters and watthour meters will be affected by the

change. These are primarily standards and development prototypes for use at power frequencies.

Most power measurements made at other, higher fre- quencies are of sufficiently low accuracy that they

should not be affected. In some cases, measurements

of transformer loss are made at a sufficient level of

accuracy that the adjustment should be made.

27
8. Transducers

Transducers and sensors designed to allow the

measurement of non-electrical quantities by electrical means generally fall in the accuracy range from 0.05%

(500 ppm) to 0.5% [21] . Those few having higher accuracies, such as load cells and platinum resistance

thermometers, quantify the measurand in terms of a

ratio of electrical quantities and therefore are unit in- dependent. Accordingly, changes in values of electri- cal standards should not create any technical problems.

However, in the case of a standard platinum re- sistance thermometer (SPRT), a small error could arise

if proper practices are not followed. An SPRT measures temperature via the ratio of its resistance at the

temperature to be measured, R^ , to that at 0°C, R^. R^

should be determined frequently as a check on both

the SPRT and the measuring instrumentation as a matter of good practice. A redetermination of R^

must be nude at the time of adjusting instrumentation to comply with Q(NIST-90) to avoid introducing a 1.69 ppm error into the ratio R,/Rq- (Alternatively,

the value of R^ can be reduced by 1.69 ppm.) The

error caused by failing to redetermine (or adjust) R^ is snuU for practical purposes, approximately -0.5 mK

at room temperature, but could be significant for the

most precise temperature measurements.

For further information on temperature measure- ments and the definition of the new 1990 temperature

scale, see Guidelines for Realizing the International Temperature Scale of 1990 (175-90) [16].

9. References

[1] B. N. Taylor, "New internationally adopted ref- erence standards of voltage and resistance," J. Res. Natl. Inst. Stand. Tech. 94, 95 (March-April

1989). (Reprinted in Appendix 3.) [2] B. N. Taylor and T. J. Witt, "New international

electrical reference standards based on the Jo- sephson and quantum Hall effects," Metrologia

26, 47 (1989).

[3] T. J. Quinn, "News from the BIPM," Metrologia

26, 69 (1989). (Reprinted in Appendix 3.)

[4] B. N. Taylor, "Report on the 17th session of the

Consultative Committee on Electricity," J. Res.

Natl. Bur. Stand. 92, 55 (Jan. - Feb. 1987). (Reprinted in Appendix 3.)

[5] B. N. Taylor, "History of the present value of

2ejh commonly used for defining national units

of voltage and possible changes in national units

of voltage and resistance," IEEE Trans. Instrum. Meas. IM-36, 659 (June 1987). (Reprinted in

Appendix 3.) [6] Bur. Intl. Poids Mes. Com. Cons. Electricite 18

(1988), to be published.

[7] R. Kaarls, B. P. Kibble, B. N. Taylor, and T. J. Witt, "Report to the Comite Consultatif d'Electricite from the Working Group on the Josephson

Effect," Bur. Intl. Poids Mes. Com. Cons.

Electricite 18 (1988), to be published. An earlier

version was published as Rapport BIPM 88/7,

August, 1988.

[8] F. Delahaye, T. Endo, O. C. Jones, V. Kose, B. N.

Taylor, and B. M. Wood, "Report to the Comite

Consultatif d'Electricite from the Working Group on the Quantum Hall Effect," Bur. Intl. Poids

Mes. Com. Cons. Electricite 18 (1988), to be published. An earlier version was published as Rapport BIPM 88/8, August, 1988.

[9] D. T. Goldman and R. J. Bell, Eds., "The International System of Units (SI)," Natl. Bur. Stand.

(U.S.), Spec. Pub. 330 (USGPO, Washington, DC,

July 1986).

[10] P. T. Olsen, R. E. Elmquist, W. D. Phillips, E. R. Williams, G. R. Jones, Jr., and V. E. Bower, "A

measurement of the NBS electrical watt in SI units," IEEE Trans. Instrum. Meas. IM-38, 238

(April 1989).

[11] W. K. Clothier, "A calculable standard of capacitance," Metrologia 1, 36 (1965).

[12] A. M. Thompson, "An absolute determination

of resistance based on a calculable standard of

capacitance," Metrologia 4, 1 (1968).

[13] B. F. Field, T. F. Finnegan, and J. Toots, "Volt

maintenance at NBS via 2e/h: A new definition

of the NBS volt," Metrologia 9, 155 (1973).

[14] M. E. Cage, R. F. Dziuba, C. T. Van Degrift, and

D. Yu, "Determination of the time dependence

of QjjBg using the quantized Hall resistance," IEEE

Trans. Instrum. Meas. IM-38, 263 (April 1989).

I

28
[15] C. Croarkin, "Measurement Assurance Programs

Part II: Development and Implementation," Natl. Bur. Stand. (U.S.), Spec. Pub. 676-11 (USGPO, [19]

Washington, DC, April 1984).

[16] B. W. Mangum and G. T. Furukawa, "Guidelines for Realizing the International Temperature Scale of 1990 (ITS-90)," to be published as [20]

Natl. Inst. Stand. Tech. Technical Note 1265.

[17] F. L. Hermach, "An Investigation of the Uncertainties of the NBS Thermal Voltage and Cur- [21] rent Converters," NBSIR 84-2903, U.S. Department of Commerce, April 1985.

[18] J. Q. Shields, R. F. Dziuba, and H. P. Layer,

"New realization of the ohm and farad using the

NBS calculable capacitor," IEEE Trans. Instrum. Meas. IM-38, 249 (April 1989).

J. D. Ramboz and R. C. McAuliff, "A Calibration

Service for Wattmeters and Watthour Meters,"

Natl. Bur. Stand. (U.S.), Tech. Note 1179 (USGPO,

Washington, DC, July 1983).

"America] National Standards Code for Electricity Metering, ANSI C12.1." (IEEE, New York,

1988).

P. S. Lederer, "Sensor Handbook for Automatic

Test, Monitoring, Diagnostics, and Control Systems Applications to Military Vehicles and Machinery," Natl. Bur. Stand. (U.S.), Spec. Pub. 615

(USGPO, Washington, DC, Oct. 1981).

29
Appendix 1

Membership of the NCSL Ad Hoc Committee 91.4

"Changes in the Volt and Ohm"

A. Anderson Guildline Instruments

L. Auxier Beckman Instruments

B. Barnaby Sandia National Laboratories

B. Belanger NIST

Chairman N. Belecki NIST

B.Bell NIST

D. Braudaway Sandia National Laboratories

S. Brodecki GTE

B. Bruce Hewlett-Packard

M. Cheniae Defense Logistics Agency

R. Cohen Rockwell International

D.Collins GTE

J. Corege Hewlett-Packard

D. Dalton John Fluke Manufacturing Co.

G. Davidson TRW Electronics & Defense Sector

A. Domenichini, Jr. Defense Logistics Agency

R.Dziuba NIST

A. Ehman Beckman Instruments

W.Eicke Self

B. Einstein Northrop Corporation

B. Field NIST

F. Hynn USAF Directorate of Metrology

R. Geesaman Hughes Aircraft Co.

Secretary R. Goff Rockwell International

J. Hirning John Huke Manufacturing Co.

S. Howie U.S. Naval Air Test Center

L. Huntley John Fluke Manufacturing Co.

I

30
Appendix 1 ( continued)

Membership of the NCSL Ad Hoc Committee 91.4

"Changes in the Volt and Ohm"

K. Jaeger Lockheed Missiles & Space Co

Publicity C.Johnston Self

F. Katzmann Ballantine Laboratories

F. Kern NASA Langley Research Center

E. Kifer Keithley Instruments

R. Kletke John Fluke Manufacturing Co.

Y. Komorita TRW Electronics & Defense Sector

J. MacKinnon U.S. Navy Western Standards Laboratory

D. Mednick Hq. Army Materiel Command

B. Moore U.S. Army TMDE Support Group

T. Mukaihata Hughes Aircraft Co.

Publicity E. Nemeroff Self N.Oldham NIST

O.Petersons NIST

J.Ramboz NIST

R. Robertson Guildline Instruments

R. Semer USAF Directorate of Metrology

M. Shaw Guildline Instruments

C. Sides Boeing Aerospace Co.

A. Smith U.S. Naval Air Test Center

S. Staley Datron Instruments

B.Taylor NIST

G. Trinite, Jr. U.S. Navy Metrology Engineering Division

A. Van Couvering Hughes Aircraft Co.

J. Wehrmeyer Eastman Kodak

B. Wood National Research Council of Canada

D. Workman Martin Marietta Aerospace

31
Appendix 2

Concerns of NCSL Ad Hoc Committee 91.4 about Logistics and Management Issues

Stemming from Changes in the Unit Representations

A. Introduction

The information given in the earUer portions of

these Guidelines deals primarily with adjustments to single standards, small groups of standards, or individual instruments to bring them into compliance

with the new unit representations. This appendix

summarizes the discussions and concerns of NCSL

Ad Hoc Committee 9L4 about problems created in

propagating the changes through a large population

of instruments or test equipment, i.e., logistics and

management issues.

Implementing the changes among such populations can create both technical and management problems since it may seriously affect calibration laboratory operations, test equipment management, and

quality-assurance activities in production areas. This

is especially probable if the metrology department

supports a large number of particularly accurate in- struments or the manufacture of exceptionally precise products. The most obvious and likely of these

problems are discussed in the ensuing paragraphs to bring them to the attention of readers. Solutions

cannot be provided since the problems are very

strongly situation-specific.

Nonetheless, it should be noted that the chance of

technical problems arising in the manufacture of products other than precision voltmeters and dc calibra- tors due to the changes is exceedingly small unless

gross blunders are made. Measurements performed

in direct support of production usually have toler- ances much larger in magnitude than those of the

changes. On the other hand, those technical problems which will come into being might well be insidi- ous rather than glaring.

B. Technical and Logistics Problems

The problems to be discussed stem from three basic causes: (1) adjustments cannot be made simultaneously throughout the entire population of stan- dards and test equipment; (2) specified values of quantities dependent upon electrical units and thus possibly requiring adjustment are imbedded in procedures, software, designs and blueprints, and in some

cases firmware; and (3) residual product manufactured in terms of the pre-1990 electrical representations will remain on the shelf until long after January

1, 1990, and perhaps long after new values are incorporated into designs.

B.l "Coexistence"

The changes will propagate throughout the National Measurement System beginning January 1, 1990. The sheer volume of instrumentation affected ensures

the existence of a transition period during which the

new and pre-January 1, 1990, representations will coexist within the System. The duration of this coexis- tence may be, in the extreme, as long as two years be- cause of the length of recalibration intervals.

Incompatible products or components — As a

result of the old and new unit representations coexisting, components or sub-assemblies manufactured to a tight tolerance in one place could be out of tolerance

at the next step in the manufacturing process if the

adjustments are not coordinated. This is especially

likely if they are manufactured by a facility or com- pany in one location for use in a different location,

such as in contractor/ sub-contractor situations. If this were to happen in a situation where sufficient

testing is done and the discrepancy is recognized immediately by people who are aware of the changes,

the cost would be minimal. On the other hand, if the

parts or subassemblies are accepted as good and the

discrepancy is sufficiently large as to cause an operational or performance problem in the long term, the

costs could be high.

Effect on instrument manufactiu:ing —One could

conceive of a situation in which a failure to coordi- nate properly the change in the volt representation

could result in the production of an instrument whose

range of adjustment is offset sufficiently that it could no longer be brought within tolerance after a few cali- bration cycles. The drawbacks of this are apparent:

the manufacturer might have to provide modifications

to fix instruments already sold (costly) and the com- pany would certainly have tarnished their reputation. Moreover, the cause of the problem might well

be (mis)perceived as a design failure by the manufac32
turer, thereby creating a large additional expense, or premature withdrawal of the product.

Coordination needed — The transition will only

occur smoothly and without incident if standards and

calibration laboratories, the test equipment management organization, the quality-assurance staff, and

the ultimate users of the test instruments and of the

data they produce cooperate to identify those situ- ations in which the coexistence of two values of the same parameter creates a problem and act in concert

to restandardize.^ All individuals involved in the use and servicing of the test equipment or instrumentation must be kept informed about the changes and

the plans for implementing them. Coordination of

the changes within a single company will not be easy;

coordination of the changes between two companies

with a supplier-user relationship will require careful

planning and extensive communication.

Calibration workload control — One of the first problems expected will surely come from partially

informed or misinformed users. Users convinced that the changes call for the complete recalibration of all instrumentation will inundate the calibration laboratory with requests for service, demanding that everything be recalibrated early in January. Of course this

is impossible, and in most cases it is probably not

necessary. Users must be informed, not only of the fact of

the changes, but of the likely impact (or lack thereof) as

well.

It will require extra effort on the part of the cali- bration laboratory and equipment management or- ganization to ensure that adjustments are made

promptly to instrumentation where the measurements

required are both critical and of high accuracy. It is advisable to perform adjustments (as described in

these Guidelines) rather than complete recalibrations

to avoid rearranging the workload pattern of the cali- bration laboratory and creating repeated peak workload levels in the future. Such a condition can cause

scheduling problems, overtime, and extra work long

after the technical effects of the changes are past.

Production support — Care must be taken that

the order in which test equipment is adjusted does

not adversely affect production or quality. There are products, such as high-accuracy instrumentation,

precision components, and guidance systems, whose

The tenn "recalibration" refers to the verification and adjustment (if necessary) of all ranges and functions of an instrument to ensure its compliance with specifications; "adjustment" or "restandardization" refers to a limited set of actions, such as adjustment of the voltage reference alone

in a digital voltmeter, intended to bring the instrument into compliance

with the new unit representations.

successful manufacture depends on compatibility of

results of measurements made by an ensemble of

instruments, perhaps in use in a number of locations or organizations within a company. The coordination of restandardization in such a situation is very

important. It must be decided whether to adjust all instrumentation at the same time or to adjust a critical subset of instruments simultaneously and proceed

with the remainder at a more leisurely pace. The

former course of action may be desirable if only to avoid possible confusion. The effect of adjusting in- strumentation providing data for control charts used

to monitor production processes must be thought out

carefully. To carry out restandardization in either

case, the full cooperation of the quality-assurance staff

is necessary since a thorough understanding of all of

the quality aspects of the system is required.

Stability test support — There is one obvious cir- cumstance in which a delay in making the changes

should be considered. In the case where components

or assemblies are in the process of stability testing,

the instrumentation used to perform the test should

not be adjusted until the conclusion of the test. If the same instrumentation is used to perform stability

measurements on groups of items whose test durations overlap, it is preferable to interrupt the test sequence at the end of the test of a particular group,

adjust the instrumentation, and begin testing afresh. This precludes any possible mix-up in the results of

testing. If instrumentation must be adjusted during

the course of a stability test due to a need to maintain

a high rate of testing, the resulting data sets will

contain offsets. There are two ways of dealing with

such effects. The first is to cope with the offset during the

analysis of the data. For limited sets of data this may

require that those data points which include an offset due to the adjustment be analyzed by a person rather

than a 'canned' computer program to avoid modifying data analysis software. For larger data sets, it may appear more feasible to build correction routines

into the analysis software. However, the problem

may be complicated by naturally occurring changes

in the measurement instruments and the methods by

which corrections for such changes are applied to the

data.

The second is to build a correction routine into the

data acquisition software. This is not recommended

because it is generally poor practice to store corrected

data. If blunders in applying corrections are made

from the beginning, reconstruction of the raw data can become very difficult, if not impossible.

33
B.2 "Embedded" Values

A potential source of trouble exists wherever volt- age values (or in rare cases resistance values) are

explicitly called out in written procedures, embedded in software, or called out in design specifications.

Whether a problem exists, and if so, how severe it is, depends on the nature of the application and, of course, the level of accuracy required. There are generally two types of situations. The first is where the

procedure or software is being used to quantify a

transfer function. In such cases one measures the

response of the circuit or device to a stimulus. This

process is in actuality a ratio determination and independent of units. Therefore no alteration needs to be

made to the values specified in the procedure or soft- ware. The calibration procedure for an instrument is a specific example of this. It describes how one determines a transfer function, i.e., the instrument's re- sponse to a known input. The table below is typical

of what might be found in a procedure for calibrating a meter. It gives the range of values to within which

the meter must be adjusted for a given input.

Input Adjust reading to be within

1.000 000 V

5.000 000 V

10.000 00 V

0.999 990 V to 1.000 010 V

4.999 990 V to 5.000 010 V

9.999 900 V to 10.000 10 V

Clearly, the ratio of the reading to the input is independent of units and therefore not affected by any

change in the volt representation. In the second type of situation, alteration of the

values specified in the procedure or software is re- quired. Such is the case where, to function properly, a device requires a specified value in a given set of

units for a particular parameter. An example of this

is the specification of alignment parameters for an

inertial guidance system based on gyroscopes or inte- grating pendula. The electrical specifications here

are driven by the physics of the guidance system.

Levels of current produced are set by inertial responses

to changes in velocity. These levels will not change

(they are like fixed-value standards in a sense) and

therefore the specified voltage and resistance values

must be adjusted to reflect the changes in the unit

representations to avoid the possibility of malfunction.

This raises the issue of what must be done when

one must maintain devices of the same model manufactured before and after January 1, 1990. The situ- ation will be confusing since alignment instructions

will be different for the two cases.

It must be stressed that there are very few instances

where accuraa/ requirements are such that specified values

in procedures, software, etc., will create a problem — the

real problem will be in mounting the effort required to identify where adjustments are necessary. B3 Residual Product

Problems may be exf)ected to be caused by the

existence in inventory of residual product manufactured in terms of the pre-1990 unit representations. This could happen at several levels: for components or sub-assemblies to be used in manufacturing

(but relatively rarely because of the level of accuracy

involved); standards guaranteed to be within a nar- row tolerance; instruments such as voltmeters and

calibrators; and systems ordered by the Department

of Defense (DoD), such as guidance systems. Pre- sumably the number of such problems will be small

since one would expect that such items would be

carefully measured or, in the case of components,

circuits adjusted before use. This should have the

effect of making the problem a calibration problem.

Again, the only solution for these problems is a wellinformed technical community. If those involved are aware of the changes, they will take action to avoid

the few problems which are likely to arise.

C. Contractual or "Legal" Problems

The problem areas mentioned above are technical

in nature; that is, they could readily result in malfunctioning equipment, erroneous measurement results, or the manufacture of unacceptable product.

There is yet another class of potential problems that could stem from contractual obligations for measure- ment traceability^ required of suppliers of technological goods by the Department of Defense, NASA,

and other government agencies; licensees of nuclear

power stations by the Nuclear Regulatory Commission; and manufacturers of medical devices by the

Food and Drug Administration. These requirements.

Traceability refers to the requirement that all measurement instruments be calibrated (periodically) and that all calibrations performed be traceable

to national standards, fundamental physical constants, or agreed-upon

standards where neither of the former two exist. That is, an unbroken

chain of documentation in the form of calibration reports referring to other

reports of calibration of the instruments and standards used, etc., showing

the linkages back to these basic starting points, must exist for each

calibration.

34
set forth in DoD MILSTD 45662A and its predecessors, and similar documents of other agencies, have

spawned a system of inspectors and audits which

pervades commerce.

In a strictly legal sense, the magnitudes of the

'practical electrical units' in the United States will

change on January 1, 1990. As noted previously,

however, all measurement equipment affected can- not possibly be adjusted instantaneously and for some

time after January 1, 1990, a fraction of the population

of instruments will remain calibrated in terms of the

pre-January 1, 1990, unit representations. This can- not be avoided unless the number of instruments in a

location is exceedingly small. Fortunately, the changes

are sufficiently small that only a very few of the processes supported by calibrated instruments will be noticeably affected technically. Nonetheless, auditors

and inspectors will no doubt take the changes very

seriously.

Accordingly, a plan for implementing the changes —complete with a rationale for continuing to use in- strumentation not yet adjusted — should be developed and documented. This is compatible with the

general philosophy behind MILSTD 45662A. The

DoD does not want to dictate the details of how to run a calibration system, but rather to describe minimum requirements and permit their contractors and

sub-contractors to deal effectively with the particulars of each situation. Control is through the requirement to document and to follow the documentation.

The following are elements which should be taken

into consideration in developing a plan:

1. Adjustment of standards according to these

Guidelines.

2. Identification of critical measurement requirements.

3. Coordination of the adjustment (or recalibration

if appropriate) of instrumentation supporting

critical measurements identified in the element

above.

4. Adjustment of all other instrumentation whose

specified accuracies are within 5 times the

changes in the unit representations.

5. Identification of instruments with accuracies

from 5 to 10 times the changes.

6. Identification of adjustment needs among these

and scheduling their adjustment.

7. Coordination of all of the above among stan- dards laboratories, calibration laboratories, test equipment management, and users.

8. Calibration report formats.

9. Use of labels, including those bearing the NCSL

'change logo' (see Fig.l, page 6). 10. Effect adjustments to standards and instruments

will have on calibration intervals, and hence on

workload cycles.

11. Effect adjustments will have on normal procedures for reporting out-of-tolerance conditions

to users.

12. Education of users of test equipment and users of

the resulting measurement data about the changes

and their likely impact.

35
Appendix 3

Reprints

We wish to thank Springer-Verlag for allowing us to reprint the article by T. J. Quinn

which appeared in Metrologia, Vol. 26, No. 1 (1989). Metrologia, the International

Journal of Scientific Metrology, is published four times a year under the auspices of the

International Committee of Weights and Measures. Metrologia publishes articles that are written by, and which constitute an important exchange of information among, the

world's experts in the science of measurement. Many of these contributions are concerned with the significant improvement of fundamental measurements in the

various fields of physics, notably the improvement of accuracy and precision in

measuring length, mass, time, electrical current, temperature, luminance, and ionizing

radiation, and the accurate determination of physical constants involved in such

measurements. Periodically, Metrologia publishes invited review articles covering

various fields of measurement, reports on international conferences of metrological

significance, and summaries of relevant research and development programs under

way in the major national standards laboratories. Its principal fields of interest are the

improvement of basic measurements in physics; improved realizations of SI units;

new values for the fundamental constants; international comparisons of standards;

and innovative metrology in general, including articles that report new methods or improvements to existing methods that contribute in a significant way to the making

of secondary measurements. Subscription inquiries may be directed to SpringerVerlag New York Inc., Service Center Secaucus, 44 Hartz Way, Secaucus, NJ 07094.

36
Volume 94, Number 2, March-April 1989 Journal of Research of the National Institute of Standards and Technology

Special Report on Electrical Standards

New Internationally Adopted Reference

Standards of Voltage and Resistance

Volume 94 Number 2 March-April 1989

B. N. Taylor

National Institute of Standards

and Technology,

Gaithersburg, MD 20899

This report provides the background

for and summarizes the main results of

the 1 8th meeting of the Consultative Committee on Electricity (CCE) of the

International Committee of Weights and Measures (CIPM) held in September

1988. Also included are the most important implications of these results. The

principal recommendations originating from the meeting, which were subse- quently adopted by the CIPM, establish new international reference standards of

voltage and resistance based on the Josephson effect and the quantum Hall

effect, respectively. The new standards, which are to come into effect starting January 1, 1990, will result in improved

uniformity of electrical measurements worldwide and their consistency with

the International System of Units or SI. To implement the CIPM recommendations in the U.S. requires that, on

January 1, 1990, the value of the U.S.

representation of the volt be increased by about 9.26 parts per million (ppm)

and the value of the U.S. representation of the ohm be increased by about 1.69 ppm. The resulting increases in the U.S.

representations of the ampere and watt

will be about 7.57 ppm and 16.84 ppm,

respectively. The CCE also recom- mended a particular method, affirmed by the CIPM, of reporting calibration

results obtained with the new reference standards that is to be used by all national standards laboratories.

Key words: CCE; CIPM; Consultative Committee on Electricity; International Committee of Weights and Measures;

International System of Units; Josephson

effect; Josephson frequency-to-voltage

quotient; ohm; quantum Hall effect; quantized Hall resistance; SI; volt.

Accepted: December 7, 1988

1. Background

The 18th meeting of the Consultative Committee on Electricity (CCE) of the International Committee of Weights and Measures (CIPM) was held

September 27 and 28, 1988, at the International Bureau of Weights and Measures (BIPM), which is located in Sevres (a suburb of Paris), France. NIST

Director E. Ambler, a member of the CIPM and

President of the CCE, chaired the meeting and the

author attended as NIST representative. Some 30

individuals from 15 countries participated.

As discussed in this journal in the author's 1987

report on the 17th meeting of the CCE held at the

BIPM m September 1986 [1], the CCE is one of

eight CIPM Consultative Committees which to- gether cover most of the areas of basic metrology.

These Committees give advice to the CIPM on

matters referred to them. They may, for example,

form "Working Groups" to study special subjects

and make specific proposals to the CIPM concerning changes in laboratory reference standards and

in the definitions of units. As organizational entities of the Treaty of the Meter, one of the responsibilities of the Consultative Committees is to ensure the

propagation and improvement of the International

System of Units or SI, the unit system used

throughout the world. The SI serves as a basis for
Volume 94, Number 2, March-April 1989 Journal of Research of the National Institute of Standards and Technology

the promotion of long-term, worldwide uniformity

of measurements which is of considerable importance to science, commerce, and industry.

However, scientific, commercial, and industrial requirements for the long-term repeatability and

worldwide consistency of voltage and resistance

measurements often exceed the accuracy with

which the SI units for such measurements, the volt' and the ohm, can be readily realized. To meet these severe demands, it is necessary to establish representations' of the volt and ohm that have a longterm reproducibility and constancy superior to the

present direct realizations of the SI units themselves. Indeed, as discussed by the author in reference

[1], in 1972 the CCE suggested that the national

standards laboratories adopt 483 594 GHzA'^ ex- actly as a conventional value of the Josephson fre- quency-to-voltage quotient for use in maintaining an accurate and reproducible representation of the

volt by means of the Josephson effect. While most

national laboratories did adopt this value, three decided to use different values. Moreover, it has be- come apparent that the CCE's 1972 value of this quotient is about 8 parts per million (ppm) smaller

than the SI value, implying that representations of

the volt based on the 1972 value are actually about

8 ppm smaller than the volt.

It has also become apparent that because most

national standards laboratories base their representation of the ohm on the mean resistance of a particular group of wire-wound resistors, the various

national representations of the ohm differ significantly from each other and the ohm, and some are

drifting excessively. Although the ThompsonLampard calculable capacitor can be used to real- ize the ohm with an uncertainty^ of less than 0.

1

ppm, it is a difficult experiment to perform routinely. Hence, the 1980 discovery of the quantum

' The volt is the SI unit of electromotive force (emf) and electric potential difference. Occasionally it may be referred to in the

literature as the absolute volt. As-maintained volt, representation of the volt, laboratory representation of the volt, "national

unit of voltage", "laboratory unit of voltage", "practical realiza- tion of the volt", and other similar terms are commonly used to indicate a "practical unit" for expressing measurement results. However, to avoid possible misunderstanding, it is best not to use the word unit in this context. The only unit of emf in the SI

is, of course, the volt. In keeping with references [2] and [3], from which this report has drawn heavily, we use the expression representation of the volt and variations thereof. The expression reference standard of voltage is also used occasionally in a

similar or related sense. The situation for the ohm and resistance

is strictly analogous.

^ Throughout, all uncertainties are meant to correspond to one

standard deviation estimates in keeping with CIPM Recommendation 1 (CI-1986) [4,5].

Hall effect (QHE) by K. von Klitzing [6] was enthusiastically welcomed by electrical metrologists

because it promised to provide a method for basing a representation of the ohm on invariant fundamental constants in direct analogy with the Josephson

effect. The QHE clearly had the potential of elimi- nating in a relatively straightforward way the

problems of nonuniformity of national representations of the ohm, their variation in time, and their inconsistency with the SI. To address the problems associated with current

national representations of the volt and ohm as dis- cussed above, the CCE at its 17th meeting

established through Declaration El (1986),^ "Concerning the Josephson effect for maintaining the

representation of the volt," the CCE Working

Group on the Josephson Effect. The CCE charged

the Working Group to propose a new value of the

Josephson frequency-to-voltage quotient consistent

with the SI value based upon all relevant data that became available by June 15, 1988. Similarly,

recognizing the rapid advances made in understanding the QHE since its comparatively recent discovery, the CCE established through Declaration E2 (1986),' "Concerning the quantum Hall ef- fect for maintaining a representation of the ohm,"

the Working Group on the Quantum Hall Effect. The CCE charged the Working Group to (i) pro- pose to the CCE, based upon all relevant data that became available by June 15, 1988, a value of the

quantized Hall resistance consistent with the SI

value for use in maintaining an accurate and stable national representation of the ohm by means of the

QHE; and (ii) develop detailed guidelines for the

proper use of the QHE to realize reliably such a

representation.*

Further, the CCE stated its intention to hold its 18th meeting in September 1988 with a view to recommending that both the proposed new value

of the Josephson frequency-to-voltage quotient and

the proposed value of the quantized Hall resistance come into effect on January 1, 1990. These values

would be used by all those national standards

' The complete declaration is given in reference [1], but see also references [5] and [7].

• The members of the CCE Working Group on the Josephson

Effect were R. Kaarls, Van Swinden Laboratorium (VSL), The

Netherlands; B. P. Kibble, National Physical Laboratory

(NPL), U.K.; B. N. Taylor, (NIST); and T. J. Witt, Coordinator (BIPM). The members of the CCE Working Group on the Quantum Hall Effect were F. Delahaye (BIPM); T. Endo, Electrotechnical Laboratory (ETL), Japan; O. C. Jones (NPL); V.

Kose, Physikalisch-Technische Bundesanstalt (PTB), F. R. G.;

B. N. Taylor, Coordinator (NIST); and B. M. Wood, National Research Council of Canada (NRCC), Canada.
Volume 94, Number 2, March-April 1989 Journal of Research of the National Institute of Standards and Technology

laboratories (and others) that base their representation of the volt on the Josephson effect, and that choose to base their representation of the ohm on

the QHE. These proposals of the CCE were subsequently approved by the CIPM [8] and by the General Conference of Weights and Measures (CGPM)

[9] under whose authority the CIPM functions. In response to the CCE's directives, each Working Group prepared a report which focused on the

review and analysis of the values of the Josephson

frequency-to-voltage quotient or quantized Hall re- sistance in SI units that were available by June 15, 1988; and the derivation of a recommended value

for the purpose of establishing an accurate and in- ternationally uniform representation of the volt and

of the ohm based on the Josephson effect and on

the quantum Hall effect, respectively. Submitted to the CCE in August 1988, the reports include useful background information as well as a discussion as

to how the new representations might be used in practice to express calibration results. In keeping

with the CCE's charge, the QHE Working Group

also prepared a companion report entitled "Technical Guidelines for the Reliable Measurement of the

Quantized Hall Resistance." Because unbiased

quantized Hall resistance determinations are re- quired for an accurate and reproducible representation of the ohm based on the QHE, these guidelines are of exceptional importance.'

2. CCE 18th Meeting Discussion and

Principal Decisions

As an aid to the reader, this section of the report

also includes some tutorial information.

2.1 Josephson Effect

2.1.1 Definition of Josephson Constant When a Josephson junction is irradiated with microwave

radiation of frequency /, its current vs voltage curve exhibits steps at highly precise quantized

Josephson voltages Uj. The voltage of the n th step

Ujin), n an integer, is related to the frequency of

the radiation by

UM)=nf/K„ (1)

' The complete reports of the Josephson and Quantum Hall Effect Working Groups including the "Technical Guidelines" (Rapports BIPM 88/77, 88/8, and 88/9) will appear in the pro- ceedings of the CCE's 18th meeting [2]. Additionally, a combined, somewhat condensed version of the two reports may be

found in reference [3] and the "Technical Guidelines" in refer- ence [10].

where ATj is commonly termed the Josephson fre- quency-to-voltage quotient [11]. The Working

Group on the Josephson Effect (WGJE) proposed

that this quotient be referred to as the Josephson

constant and, since no symbol had yet been

adopted for it, that it be denoted by Kj. It follows

from eq (1) that the Josephson constant is equal to the frequency-to-voltage quotient of the « = 1 step. The theory of the Josephson effect predicts, and

the experimentally observed universality of eq (1)

is consistent with the prediction, that ATj is equal to the invariant quotient of fundamental constants 2e/h, where e is the elementary charge and h is the

Planck constant [11]. For the purpose of including

data from measurements of fundamental constants

in the derivation of their recommended value of ^j,

the WGJE assumed that 2e/h =K}. However, ATj is not intended to represent the combination of fundamental constants 2e/h.

2.1.2 Josephson Effect Reference Standard of

Voltage The CCE reviewed the report from the

WGJE and discussed at some length the draft rec- ommendation El (1988), "Representation of the

volt by means of the Josephson effect," prepared

jointly by the WGJE and the Working Group on

the Quantum Hall Effect. The CCE then agreed:

(i) to use the term "Josephson constant" with

symbol Kj to denote the Josephson frequency-tovoltage quotient;

(ii) to accept the WGJE's recommended value of

Kj, namely, A:j=(483 597.9 ±0.2) GHzA^, where

the 0.2 GHz/V assigned one-standard-deviation

uncertainty corresponds to a relative uncertainty of

0.4 ppm;

(iii) to use this recommended value to define a conventional value of Kj and to denote it by the

H*>f symbol Kj_go, so that /i:j_9o=483 597.9 GHzA^ exactly. (The subscript 90 derives from the fact that

this new conventional value of the Josephson constant is to come into effect starting January 1, 1990, a date reaffirmed by the CCE.) The CCE also noted

(iv) that since Arj_9o exceeds the CCE's 1972 conventional value of the Josephson constant by 3.9 GHzA'' or about 8.065 ppm, the new representation of the volt will exceed that based on the 1972

value by about 8.065 ppm; and further agreed

(v) that because the purpose of the new volt rep- resentation is to improve the worldwide uniformity

of voltage measurements and their consistency

with the SI, laboratories which do not base their national representation of the volt on the Joseph-
Volume 94, Number 2, March-April 1989 Journal of Research of the National Institute of Standards and Technology

son effect should, on January 1, 1990, adjust the

value of their national volt representation so that it

is consistent with the new representation. Further,

this consistency should be maintained by having a transportable voltage standard periodically cali- brated by a laboratory that does base its representation of the volt on the Josephson effect;

(vi) that even if future, more accurate measure- ments of Kj indicate that the recommended value

differs from the SI value by some small amount,

the conventional value Kj_9o should not be altered. Rather, the CCE could simply note the difference

between a representation of the volt based on Arj_9o

and the volt; and

(vii) that because an accurate representation of

the volt is important to science, commerce, and in- dustry, laboratories should continue their efforts to

realize the volt with greater accuracy, either di- rectly or indirectly via measurements of fundamental constants. This could lead to a significant

reduction in the uncertainty assigned to the new

volt representation.

Having concurred on these points, the CCE edited the draft recommendation El (1988) to bring it to final form. The following week it was submitted

to the CIPM for approval at its 77th meeting held on October 4-6, 1988, at the BIPM. After some

minor editorial changes, the CIPM adopted it as its own recommendation [12]. The following is the

English language version (the French language

version is the official one and is given in references

[2] and [12]):

Representation of the Volt by Means of the

Josephson Effect

Recommendation 1 (CI-1988)

The Comite International des Poids et Mesures,

acting in accordance with instructions given in Resolution 6 of the 18th Conference Generale des

Poids et Mesures concerning the forthcoming adjustment of the representations of the volt and the

ohm,

considering —that a detailed study of the results of the most

recent determinations leads to a value of 483 597.9

GHz/V for the Josephson constant, Kj, that is to

say, for the quotient of frequency divided by the

potential difference corresponding to the « = I step

in the Josephson effect, —that the Josephson effect together with this value of K] can be used to establish a reference

standard of electromotive force having a one-stan- dard-deviation uncertainty with respect to the volt

estimated to be 4 parts in 10^ and a reproducibility

which is significantly better,

recommends

—that 483 597.9 GHzA" exactly be adopted as a conventional value, denoted by K]_go, for the

Josephson constant, Kj, —that this new value be used from 1st January

1990, and not before, to replace the values currently in use, —that this new value be used from this same

date by all laboratories which base their measure- ments of electromotive force on the Josephson ef- fect, and —that from this same date all other laboratories

adjust the value of their laboratory reference stan- dards to agree with the new adopted value,

is of the opinion —that no change in this recommended value of

the Josephson constant will be necessary in the

foreseeable future, and

draws the attention of laboratories to the fact that the new value is greater by 3.9 GHzA^, or about 8 parts in 10', than the value given in 1972 by the

Comite Consultatif d'Electricite in its Declaration

E-72.

2.2 Quantum Hall Effect

2.2.1 Definition of the von Klitzing Constant The

QHE is characteristic of certain high mobility

semiconductor devices of standard Hall-bar ge- ometry when in a large applied magnetic field and

cooled to a temperature of about one kelvin. For a

fixed current / through a QHE device there are regions in the curve of Hall voltage vs gate

voltage, or of Hall voltage vs magnetic field depending upon the device, where the Hall voltage

C/h remains constant as the gate voltage or magnetic field is varied. These regions of constant Hall

voltage are termed Hall plateaus. Under the proper

experimental conditions, the Hall resistance of the

ith plateau i?H(0. defined as the quotient of the

Hall voltage of the ith plateau to the current /, is given by

Ruii)=Un{i)/I=Rvi/i, (2)

where / is an integer [13]. Because /?h(') is often

referred to as the quantized Hall resistance regardless of plateau number, the Working Group on the

Quantum Hall Effect (WGQHE) proposed that to avoid confusion, the symbol -Rr be used as the Hall

voltage-to-current quotient or resistance of the

/ = 1 plateau and that it be termed the von Klitzing constant after the discoverer of the QHE. It thus

follows from eq (2) that /?k=^h(1)-
Volume 94, Number 2, March-April 1989 Journal of Research of the National Institute of Standards and Technology

The theory of the QHE predicts, and the experimentally observed universality of eq (2) is consis- tent with the prediction, that /?k is equal to the

invariant quotient of fundamental constants h/e^

[13]. For the purpose of including data from measurements of fundamental constants in the derivation of their recommended value of Ry^, the

WGQHE assumed that A/e^=/?K- However, in analogy with Kj, /?k is not intended to represent

the combination of fundamental constants h/e^.

2.2.2 Quantum Hall Effect Reference Standard of

Resistance The CCE reviewed the report of the

WGQHE and discussed the draft recommendation

E2 (1988), "Representation of the ohm by means of

the quantum Hall effect," prepared jointly by the

two Working Groups. Because of the similarities between the QHE and the Josephson effect, the

review and discussion proceeded expeditiously. In- deed, the second half of point (iii) as given here in section 2.1.2 on the Josephson effect and all of

points (v), (vi), and (vii) were viewed by the CCE

as applying to the quantum Hall effect as well. Also in analogy with the Josephson effect, the

CCE agreed:

(i) to use the term "von Klitzing constant" with

symbol i?K to denote the Hall voltage to current

quotient or resistance of the / = 1 plateau;

(ii) to accept the WGQHE's recommended value

of /?K, namely, /?k = (25 812.807±0.005) ft, where

the 0.005 ft assigned one-standard-deviation uncertainty corresponds to a relative uncertainty of 0.2 ppm; and

(iii) to use this recommended value to define a conventional value of /?k and to denote it by the

def symbol /?k-9o, so that i?K-9o=25 812.807 ft exactly.

The same procedure was followed for draft rec- ommendation E2 (1988) as for El (1988) regarding

the Josephson effect. The final CIPM English lan- guage version is as follows:

Representation of the Ohm by Means of the

Quantum Hall Effect

Recommendation 2 (CI-1988)

The Comite International des Poids et Mesures,

acting in accordance with instructions given in Resolution 6 of the 18th Conference Generale des

Poids et Mesures concerning the forthcoming adjustment of the representations of the volt and the

ohm,

considering —that most existing laboratory reference stan- dards of resistance change significantly with time, —that a laboratory reference standard of resis- tance based on the quantum Hall effect would be

stable and reproducible, —that a detailed study of the results of the most

recent determinations leads to a value of 25 812.807

ft for the von Klitzing constant, /?k:> that is to say, for the quotient of the Hall potential difference di- vided by current corresponding to the plateau / = 1 in the quantum Hall effect, —that the quantum Hall effect, together with

this value of /?k, can be used to establish a refer- ence standard of resistance having a one-standarddeviation uncertainty with respect to the ohm

estimated to be 2 parts in 10', and a reproducibility

which is significantly better,

recommends —that 25 812.807 ft exactly be adopted as a conventional value, denoted by i?K-9o» for the von Klitzing constant, i?K. —that this value be used from 1st January 1990,

and not before, by all laboratories which base their measurements of resistance on the quantum Hall

effect, —that from this same date all other laboratories

adjust the value of their laboratory reference stan- dards to agree with i?K-9o. —that in the use of the quantum Hall effect to

establish a laboratory reference standard of resis- tance, laboratories follow the most recent edition

of the "Technical Guidelines for Reliable Measurements of the Quantized Hall Resistance" drawn up

by the Comite Consultatif d'Electricite and published by the Bureau International des Poids et Mesures,

and is of the opinion —that no change in this recommended value of

the von Klitzing constant will be necessary in the

foreseeable future.

2.3 Practical Implementation of Recommendations

As implied by the discussion of section 1, the

results of voltage and resistance measurements ex- pressed in terms of representations of the volt and

ohm based on the Josephson and quantum Hall ef- fects, respectively, will have a higher precision

than the same measurement results expressed in terms of the volt and ohm themselves. Indeed, this

is one of the principal reasons for establishing such
Volume 94, Number 2, March-April 1989 Journal of Research of the National Institute of Standards and Technology

representations/ The question arises, however, as to how such measurement results should be re- ported in practice. The Working Groups recognized that the potential for significant confusion

internationally could best be eliminated by having

each national standards laboratory adopt the same

approach. To this end, in their reports the Working

Groups identified and considered the advantages

and disadvantages of three different approaches to the reporting problem, two of which are both rig- orous and correct [2]. In the first, new "practical

units" "V90" and "figo" are defined; in the second,

new, so-called "conventional physical quantities" for electromotive force (and electric potential dif- ference) and resistance, "£"90" and "Rgo," are defined. The CCE discussed at length the three ap- proaches identified by the Working Groups and

concluded that there was an alternative solution,

similar to the Working Groups' third approach,

that is also rigorous but avoids

(i) defining new practical units of emf and resis- tance that are likely to differ from the volt and ohm

by small amounts and which would be parallel to and thus in competition with the volt and ohm.

(Defining such units automatically leads to practi- cal electrical units for current, power, capacitance,

etc., thereby giving the appearance that a complete new system of electrical units has been established

outside of the SI.) The CCE's alternative solution

also avoids

(ii) defining new conventional physical quantities for emf and resistance which are likely to differ from traditional or true emf and resistance by small

amounts. (Defining such quantities automatically

leads to conventional physical quantities for current, power, capacitance, etc.; and to the peculiar

situation of, for example, the same standard cell having both a conventional emf and a true emf.)

Further, the alternative solution avoids

(iii) the use of subscripts or other distinguishing

symbols of any sort on either unit symbols or quantity symbols. (With the elimination of such subscripts and symbols, for example, those denoting

particular laboratories or dates, the national stan- dards laboratories can avoid giving the impression

' As noted by the CCE [2], the Josephson and quantum Hall

effects and the values Ki-90 and Rk-9o cannot be used to define the volt and ohm. To do so would require a change in the status of the permeability of vacuum ;xo from an exactly defined constant, thereby abrogating the definition of the ampere. It would

also give rise to electrical units which would be incompatible with the definition of the kilogram and units derived from it.

to the users of their calibration services that there

is more than one representation of the volt and of

the ohm in general use, that there may be signifi- cant differences among national realizations of the new volt and ohm representations, and that either

the national realizations or the new representations

differ significantly from the SI.)

The CCE's solution, which was affirmed by the

CIPM at its 77th meeting [12] and which all national standards laboratories are requested to fol- low, is indicated in the following variation of the

example given by the CCE [2] (the treatment of

resistance measurements is strictly analogous):

The emf E of an unknown standard cell cali- brated in terms of a representation of the volt based on the Josephson effect and the conventional value

of the Josephson constant Kj_go, may be rigorously

expressed in terms of the (SI) volt V as (to be

specific):

£: =(1.018 123 45) V±e, (3)

where € represents the total uncertainty, in volts,

and is composed of the following two components:

AjE, the combined uncertainty associated with the

calibration itself and with the realization of the

Josephson effect volt representation at the particular standards laboratory performing the calibration; and A/1 , the uncertainty with which the ratio Kj_go/K3 is known (i.e., it is assumed that Kj_go/

^j=l±A/4). According to Recommendation 1 (CI-1988), A^ is 4 parts in 10' or 0.4 ppm (assigned one standard deviation).

Since, by international agreement, AA is com- mon to all laboratories, the two uncertainties AE

and AA need not be formally combined to obtain

the total uncertainty e but may be separately indi- cated. Hence, the measured emf E may be ex- pressed as £ =(1.018 123 45) V±A£ (4)

for all practical purposes of precision electrical metrology and trade, with AA appearing separately on the calibration certificate when the precision of

the calibration warrants it. If, for example, AE/E is significantly greater than 0.4 ppm, AA may be

omitted with negligible effect. An example of the wording that might be used on a NIST Report of Calibration for a standard cell enclosure for the case where AA may not be omitted and which is a variation of the wording given

in an example developed by the CCE [2], is as fol- lows:
Volume 94, Number 2, March-April 1989 Journal of Research of the National Institute of Standards and Technology

Sample Hypothetical NIST Calibration Report

This standard cell enclosure was received (date) under power at its normal operating temperature.

The values given in the table below are based on the results of daily

measurements of the differences between the emfs of the cells in this standard and those of NIST working standards calibrated in terms of

the Josephson effect using the new conventional value of the Josephson

constant internationally adopted for use starting January 1, 1990 (see

Note A). The measurements were made in the period from (date) to

(date).

Cell emf Uncertainty

number (volts, V) (microvolts, /iV)

1 1.018 119 85 0.27 2 1.018 133 77 0.27

3 1.018 126 42 0.27

4 1.018 141 53 0.27

(Information relating to the measurements and their uncertainties to be

given here.)

Note A

The value of the Josephson constant used in this calibration, namely,

Arj_9o=483 597.9 GHzA'^ exactly, is that adopted by international agree- ment for implementation starting on January 1, 1990, by all national

standards laboratories that base their national representation of the volt

(i.e., their national "practical unit" of voltage) on the Josephson effect. Since all such laboratories now use the same conventional value of the

Josephson constant while prior to this date several different values were

in use, the significant differences which previously existed among the

values of some national representations of the volt no longer exist. Moreover, the national standards laboratories of those countries that do

not use the Josephson effect for this purpose are requested to maintain

their own national representation of the volt so as to be consistent with

the above conventional value of the Josephson constant, for example,

through periodic comparisons with a laboratory that does use the

Josephson effect. An ideal representation of the volt based on the

Josephson effect and Kj_9ois expected to be consistent with the volt as defined in the International System of Units (SI) to within an assigned

relative one-standard-deviation uncertainty of 0.4 ppm (0.41 juV for an emf of 1.018 V). Because this uncertainty is the same for all national

standards laboratories, it has not been formally included in the uncertainties given in the table. However, its existence must be taken into

account when the utmost consistency between electrical and nonelectrical measurements of the same physical quantity is required.

2.4 Future Work on Electrical Units Hall effect, led the CCE to adopt the following

formal recommendation which was also approved The ideas agreed upon by the CCE as given in point (vii) in Sect. 2.1.2 on the Josephson effect,

and which apply equally as well to the quantum

by the CIPM at its 77th meeting [12].
Volume 94, Number 2, March-April 1989 Journal of Research of the National Institute of Standards and Technology

Realization of the Electrical SI Units

Recommendation E3 (1988)

The Comite Consultatif d'Electricite

recognizing —the importance to science, commerce and in- dustry of accuracy in electrical measurements, —the fact that this accuracy depends on the ac- curacy of the reference standards of the electrical

units, —the very close ties that now exist between

electrical metrology and fundamental physical constants, —the possibility of obtaining more accurate ref- erence standards of the electrical units either di- rectly from the realizations of their definitions or

indirectly from measurements of fundamental constants, and —the continuing need to compare among themselves independent realizations of the units and in- dependent measurements of fundamental constants

to verify their accuracy,

recommends —that laboratories continue their work on the

electrical units by undertaking direct realizations of

these units and measurements of the fundamental

constants, and —that laboratories pursue the improvement of

the means for the international comparison of national standards of electromotive force and electri- cal resistance.

3. Conclusion

The apparatus currently being used by the national standards laboratories is such that the total experimental uncertainty associated with a particular national representation of the volt based on the

Josephson effect generally lies in the range 0.01 to

0.2 ppm. As a consequence, with the worldwide

adoption starting January 1, 1990, of the new conventional value of the Josephson constant Arj_9o, all national representations of the volt should be

equivalent to within a few tenths of a ppm. Similarly, the total experimental uncertainty associated

with the measurement of quantized Hall resistances

also generally lies in the range 0.01 to 0.2 ppm.

Hence, with the worldwide adoption starting on

January 1, 1990, of a new representation of the

ohm based on the QHE and the conventional value

of the von Klitzing constant i?K-9o. all national rep- resentations of the ohm should also be equivalent

to within a few tenths of a ppm. Moreover, these new national volt and ohm representations should

be consistent with the volt and the ohm to better than 0.5 ppm.

In the U.S., the value of the present national rep- resentation of the volt maintained by NIST will need to be increased on January 1, 1990, by about

9.26 ppm to bring it into agreement with the new

representation of the volt. This is sufficiently large

that literally thousands of electrical standards,

measuring instruments, and electronic systems

throughout the Nation will have to be adjusted or

recalibrated in order to conform with the new representation. Most other countries will be required

to make a similar change in the value of their present representation of the volt as can be seen from figure 1. On the same date, the value of the U.S. representation of the ohm maintained by

NIST will need to be increased by about 1.69 ppm

to bring it into agreement with the new representation of the ohm based on the quantum Hall effect. This too is an amount which is of significance to many existing standards, instruments, and systems.

1

-1

-2 -

-3 -

-4

,vsv .>.XXNVV«.\\VXV\V>.XXVX^V>.>XWV\VXWXNW>.\X>.WV

E

O. c -

+4.500 ppm

-6 —

-7 +1.323 ppm

-8 -1.199 ppm

-9 VNVS

-10 —

NEW VOLT

REPRESENTATION

U.S.S.R.

FRANCE

Oppm

-3.565 ppm

-6.741 ppm

ALL OTHER q naa „r„^ COUNTRIES -8°65 PP*"

U.S. -9.264 ppm

Figure 1. Graphical comparison of the value of the present rep- resentation of the volt of various countries as based on the Josephson effect, with the new representation of the volt based on the Josephson effect and the CIPM conventional value of the Josephson constant Kj-go which is to come into effect starting on January 1, 1990. The value of the volt representation indi- cated by "All Other Countries" is based on the conventional

value of the Josephson constant stated by the CCE in 1972, namely, 483 594 GHzA^. The countries that currently use this value include Australia, Canada, Finland, F.R.G., G.D.R.,

Italy, Japan, The Netherlands, and the U.K. The BIPM uses this value as well, but NIST uses 483 593.420 GHzA^. Thus, as the

figure shows, on January 1, 1990, the value of the present U.S.

volt representation will need to be increased by 9.264 ppm to bring it into conformity with the new representation.
Volume 94, Number 2, March-April 1989 Journal of Research of the National Institute of Standards and Technology

The change required in the value of the national

representation of the ohm of other countries varies

betvi'een a decrease of a few tenths of a ppm to an

increase in excess of 3 ppm.

Since A=V/il where A is the ampere as defined

in the SI; and W=:VVn where W is the watt as defined in the SI, the 9.264 ppm and 1.69 ppm in- crease in the U.S. representation of the volt and of

the ohm, respectively, imply that on January 1, 1990, (i) the U.S. representation of the ampere will increase by about 7.57 ppm and (ii) the U.S. electri- cal representation of the watt will increase by

about 16.84 ppm. Because an ideal volt representation based on the Josephson effect and Kj_go is ex- pected to be consistent with the volt to within an

assigned relative one-standard-deviation uncertainty of 0.4 ppm; and an ideal ohm representation

based on the QHE and /?k-9o is expected to be consistent with the ohm to within an assigned one-stan- dard-deviation uncertainty of 0.2 ppm, ampere and

watt representations derived from such ideal volt and ohm representations via the above equations

are expected to be consistent with the ampere and

watt to within a one-standard-deviation uncertainty of 0.45 ppm and 0.83 ppm, respectively.

The CCE strongly believes, and the author fully concurs, that the significant improvement in the in- ternational uniformity of electrical measurements

and their consistency with the SI which will result from implementing the new representations of the

volt and ohm will be of major benefit to science,

commerce, and industry throughout the world; and

that the costs associated with implementing the new representations will be far outweighed by

these benefits.

[6] V. Klitzing, K., Dorda, G., and Pepper, M., Phys. Rev.

Lett. 45, 494 (1980).

[7] BIPM Com. Cons. Electricite 17, E6, E88; ElO, E92

(1986).

[8] Reference [4], p. 10.

[9] BIPM Comptes Rendus 18'^ Conf. Gen. Poids et Mesures,

p. 100 (1987). See also Giacomo, P., Metrologia 25, 113

(1988).

[10] Delahaye, F., Metrologia 26, No. 1 (1989), to be published.

[11] Clarke, J., Am. J. Phys. 38, 1071 (1970).

[12] BIPM Proc.-Verb. Com. Int. Poids et Mesures 56 (1988),

to be published.

[13] The Quantum Hall Effect, eds. R. E. Prange and S. M.

Girvin, Springer-Verlag, NY (1987). This book provides a comprehensive review of the quantum Hall effect.

About the author: Barry N. Taylor, a physicist, is head of the Fundamental Constants Data Center in the NIST National Measurement Laboratory and

Chief Editor of the Journal of Research of the National Institute of Standards and Technology.

4. References

[1] Taylor, B. N., J. Res. Natl. Bur. Stand. 92, 55 (1987).

[2] BIPM Com. Cons. Electricite 18 (1988), to be published.

[3] Taylor, B. N., and Witt, T. J., Metrologia 26, No. 1 (1989), to be published.

[4] BIPM Proc.-Verb. Com. Int. Poids et Mesures 54, 35

(1986). See also Kaarls, R., BIPM Proc. Verb. Int. Poids et Mesures 49, Al, 26 (1981); and Giacomo, P., Metrologia

18, 41 (1982).

[5] Giacomo, P., Metrologia 24, 45 (1987).

Metrologia 26, 69-74 (1989) metrologia

© Springer-Verlag 1989

News from the BIPM

T. J. Quinn

Bureau International des Poids et Mesures, Pavilion de Breteuil, F-92312 Sevres Cedex, France

Received: October 14, 1988

Comite International des Poids et Mesures

77th Meeting

The Comite International des Poids et Mesures

(CIPM) at its 77th Meeting, held at the Pavilion de

Breteuil on the 4th, 5th and 6th of October 1988,

adopted two important Recommendations concerning the use of the Josephson effect and the quantum

Hall effect for maintaining reference standards for the measurement of emf and resistance. These Recommendations are based upon proposals made to the CIPM

by its Comite Consultatif d' Electricite (CCE) which

met in September 1988. The CCE proposals were the

result of a great deal of work and discussion that had

taken place among representatives of the national

standards laboratories and the BIPM, particularly over the past twelve months.

New determinations of the SI volt and ohm, directly by realizations of the SI definitions and indirectly

through determinations of the relevant fundamental

physical constants, have established the values of the Josephson constant K, and the von Klitzing constant

i?K of the quantum Hall effect with uncertainties of a few parts in 10^. The constants K, and i?K were as- sumed by the CCE to be equal to 2 e/h and /j/e^ respectively for the purpose of including determinations of

fundamental constants in their evaluation. The reproducibility, however, of the emf across a Josephson

junction and the resistance of a quantum Hall device

is much better than a few parts in 10' and approaches one part in 10^. Furthermore, the value of 2 e/h sug- gested by the CCE in 1972 is now known to be in error by about 8 parts in 10^, although not all national

standards laboratories are using this value. In order to unify world reference standards of emf

and allow users to take advantage of the great repro- ducibihty of the Josephson effect and the quantum

Hall effect, the CIPM acted on the advice of the CCE

and adopted particular values of Kj and Ry^, designated Kj_gQ and ^k 9o> for use by all laboratories

beginning on 1st January 1990. For those countries

that have based their reference standard of emf on the Josephson junction and use the 1972 CCE value for 2 e/h, the adoption of the new value for Kj will lead to a change in their reference standards of about 8 parts

in 10^, i. e. 8 |iV per volt. For the countries using values

of Kj which differ from the 1972 CCE value, the changes will be within the range of about 9 to 3.5 parts

in 10^

Also 1st January 1990 it is expected that a new

international temperature scale, the International

Temperature Scale of 1990 (ITS-90), will come into

effect, which will replace the International Practical

Temperature Scale of 1968 (IPTS-68). The differences between ITS-90 and IPTS-68 are significant; not only

are the scales substantially different in definition but

temperatures measured on the ITS-90 differ significantly from those measured on the IPTS-68. For ex- ample, near room temperature: for tgg — 20 °C the dif- ference T90 — Tgg - - 5 mK and for t^g^lOO °C the

difference T90 — Tgs ^ — 25 mK. The normal boiling

point of water thus will be about 99.97 "C and not

100.00 °C as in the past.

It is important that users be made aware of these changes in electrical and temperature reference stan- dards well in advance of their introduction. For this reason the 18th Conference Generale des Poids et Measures asked the CIPM to announce one year in advance, namely at the beginning of 1989, the magnitude of the changes consequent upon their coming into

effect. These official announcements are now made in the following three CIPM recommendations:

Representation of the volt by means

of the Josephson effect

Recommendation 1 (CI-1988)

The Comite International des Poids et Mesures,

acting in accordance with instructions given in Resolution 6 of the 1 8th Conference Generale des Poids et
Mesures concerning the forthcoming adjustment of

the representations of the volt and the ohm,

considering - that a detailed study of the results of the most recent determinations leads to a value of

483 597.9 GHz/V for the Josephson constant, /sTj, that

is to say, for the quotient of frequency divided by the

potential difference corresponding to the n — 1 step in the Josephson effect, - that the Josephson effect together with this value

of Ki can be used to establish a reference standard of

electromotive force having a one-standard-deviation

uncertainty with respect to the volt estimated to be

4 parts in 10^, and a reproducibility which is significantly better,

recommends - that 483 597.9 GHz/V exactly be adopted as a conventional value, denoted by ATj.go, for the Joseph- son constant, ATj, - that this new value be used from 1 st January 1990, and not before, to replace the values

currently in use, - that this new value be used from this same date

by all laboratories which base their measurements of

electromotive force on the Josephson effect, and

- that from this same date all other laboratories

adjust the value of their laboratory reference stan- dards to agree with the new adopted value,

is of the opinion - that no change in this recommended value of the Josephson constant will be necessary in the foresee- able future, and

draws the attention of laboratories to the fact that the new value is greater by 3.9 GHz/V, or about 8 parts in 10^, than the value given in 1972 by the Comite Consultatif d 'Electricite in its Declaration E-72.

Representation of the ohm by means

of the quantum Hall effect

Recommendation 2 (CI-1988)

The Comite International des Poids et Mesures,

acting in accordance with instructions given in Resolution 6 of the 1 8th Conference Generale des

Poids et Mesures concerning the forthcoming adjustment of the representations of the volt and the ohm,

considering - that most existing laboratory reference standards

of resistance change significantly with time,

- that a laboratory reference standard of resistance

based on the quantum Hall effect would be stable and

reproducible, - that a detailed study of the results of the most

recent determinations leads to a value of 25 812.807 Q

for the von Klitzing constant, i?^, that is to say, for the quotient of the Hall potential difference divided

by current corresponding to the plateau i = 1 in the quantum Hall effect, - that the quantum Hall effect, together with this value of Ry^, can be used to establish a reference stan- dard of resistance having a one-standard-deviation

uncertainty with respect to the ohm estimated to be

2 parts in 10^, and a reproducibility which is significantly better,

recommends

-that 25 81 2.807 Q exactly be adopted as a conventional value, denoted by Rk-90' for the von Klit- zing constant, /?k. -that this value be used from 1st January 1990,

and not before, by all laboratories which base their measurements of resistance on the quantum Hall effect, - that from this same date all other laboratories

adjust the value of their laboratory reference stan- dards to agree with i?K-9o> - that in the use of the quantum Hall effect to establish a laboratory reference standard of resistance,

laboratories follow the most recent edition of the

technical guidelines for reliable measurements of the quantized Hall resistance drawn up by the Comite

Consultatif d 'Electricite and published by the Bureau

International des Poids et Mesures, and

is of the opinion - that no change in this recommended value of the von Klitzing constant will be necessary in the foresee- able future.

Preparation of the International Temperature Scale

of 1990 (ITS-90)

Recommendation 3 (CI-1988)

The Comite International des Poids et Mesures,

acting in accordance with instructions given in Resolution 7 of the 1 8th Conference Generale des

Poids et Mesures concerning the preparation of the new international temperature scale, announces that the differences between the ITS-90 and

the IPTS-68 will be approximately those indicated in the graph attached to this Recommendation,

recommends that national laboratories take note of

these differences with a view to the implementation of

the ITS-90 on 1st January 1990.
0.02-

oo

^•'^CO(O 4w -0.02

1o

-0.04

a>o

c

0)0) •^•a

o>

s.3*

(0

0)a

E -0.2

-200

Fig. 1. Graph of tgg- fgg

1000

Comite Consultatif d'Electricite

18th Meeting

The CCE at its meeting in September 1988 considered very carefully the way in which the recommended

values for Kj and i^^: (defined above) be used. Al- though no formal recommendation was made on this point, the following four statements were drafted and

unanimously approved by the CCE and subsequently

supported by the CIPM ^

LL Recommendations 1 (CI-1988) and 2 (€1-1988)

do not constitute a redefinition of SI units The conventional values Kj_9o and i?K-9o cannot be

used as bases for defining the volt and ohm (meaning

the present volt and ohm units in the Systeme Inter- national d'Unites). To do so would change the status of //q from that of a constant having an exactly defined

value (and would therefore abrogate the definition of

the ampere) and would also produce electrical units

which would be incompatible with the definition of the kilogram and units derived from it.

' Because the original draft CCE recommendations were slighly modified by the CIPM, I have chosen to refer here only to the CIPM designations, i.e., CI-1988.

i.2. Concerning the use of subscripts on symbols

for quantities or units The CCE considers that symbols for electromotive

force (electric potential, electric potential difference)

and electric resistance, and for the volt and the ohm,

should not be modified by adding suffixes to denote

particular laboratories or dates. The principal reasons for this viewpoint with re- spect to the physical quantities are that

- until now, temperature being one of the very few

exceptions, it has not been necessary to introduce explicitly the concept of a system of conventional physical quantities differing from the traditional quantities,-it would be difficult to make such a concept

widely understood and accepted, - the concept, if introduced for electromotive force

and electrical resistance, would propagate to other

quantities.

The principal reasons for this viewpoint with re- spect to the units are that: - the appearance of creating a unit system other

than SI should be avoided, particularly as this would

propagate to units for other quantities,
- the new reference standards will be completely

satisfactory representations of the volt and the ohm

for the great majority of applications,

-any disagreement between those laboratories

that realize the new reference standards will be negligible from the point of view of the great majority of

users,- many countries are in any case constrained by

their existing legislation concerning physical quantities and units to use the SI names and symbols.

1.3. Concerning the practical implementation of the Recommendations 1 (CI-1988) and 2 (€1-1988)

The CCE having carefully considered the three ap- proaches listed in the reports of the Working Groups,

documents CCE/88-34 and CCE/88-35 ^ is of the opinion that a rigorous solution to this problem has

been identified which avoids

(i) defining new units "V90" or "Q90",

(ii) defining new physical quantities "Ego" or "R90" •

(iii) the use of subscripts or other distinguishing symbols of any sort on either unit symbols or quantity

symbols.

The preferred approach is indicated in the following

example of a statement that may be communicated to users of standard-cell calibration certificates

The measured emf, E, or electric potential differ- ence, IJ, of the unknown source may be rigorously

expressed in terms of the SI volt, V, as

£ = (1.018 XXX xx)V + £ The symbol e represents the total uncertainty, at the

level of one standard deviation, and is given by

E = [{^Ef + {EhY]'|^

where A£ is the combined uncertainty in volts (at one

standard deviation) associated with the calibration it- self and with the realization of the Josephson-effect

reference standard at the particular national standards

laboratory, and 5 represents the relative uncertainty

with which the ratio Kj.go/Kj is known. At present 5

is estimated to be 4 x 10"^ (one standard deviation)

according to Recommendation 1 (CI-1988).

Since, by international agreement, 5 is common to

all laboratories, it may be indicated separately and the above expression for E may be rewritten

£ = (1.018 XXX xx)V±A£

for all practical purposes of precision electrical metrology and trade. However, when this is done, 5 should

^ Note to reader: these two Documents dealt with the Joseph- son effect and the quantum Hall effect, respectively.

appear separately on the certificate where the precision is such as to require it. When A£/£ is significantly

greater than 4 x 10"', 5 may be omitted.

The treatment of resistance measurements [Recommendation 2 (CI-1988)] is strictly analogous.

1.4. Example of the wording to be used on calibration certificates The values of emf below are based on ... [a description of the calibration procedure may be placed

here] ... using the new conventional value of the Josephson constant internationally adopted for use from 1st January 1990 (see Note A).

cell number emf in volts uncertainty in volts

1 1.018 123 4 A£

[other data related to the calibration may be placed

here]

Note A

The value of the Josephson constant used in this cali- bration is /Cj_9o = 483 597.9 GHz/V and is that adopted, by international agreement, for implementation on 1st January 1990 by all national standards

laboratories that use the Josephson effect as a refer- ence standard of the volt. By international agreement,

all such laboratories now use the same value of the Josephson constant whereas until recently they did

not. National standards laboratories of those countries that do not use the Josephson effect as a reference standard can maintain their own reference standards

so as to be consistent with the above value of the Josephson constant, by periodic comparisons with a

laboratory that does use the Josephson effect. An ideal reference standard of emf based on the Josephson ef- fect and X,_9o is consistent with the SI volt within an

assigned fractional one-standard-deviation uncertainty of 4 X 10"' (0.4 |iV for an emf of one volt). Because this uncertainty is the same for all national

standards laboratories, it has not been formally in- cluded in the uncertainties given in the table. However,

its existence must be recognized when the utmost

consistency between electrical and mechanical measurement is required.

Comite Consultatif pour la Masse

et les grandeurs apparentees

3rd Meeting

The Comite Consultatif pour la Masse et les gran- deurs apparentees (CCM) met at the Pavilion de Breteuil on the 26th and 27th of May 1988. The President,

Prof Bray, being indisposed the meeting was chaired

by Dr. Giacomo, Director of the BIPM. The CCM

covers a wide range of activities in the fields of mass,
force and pressure standards and much of the meeting was devoted to the examination of reports drawn up

by the nine Working Groups. Most of these Working

Groups had met in the days immediately preceding the meeting of the CCM. The fields of work covered by

the Working Groups were: 1, the measurement of air density; 2, the conservation and behaviour of Pt/Ir mass standards; 3, the conservation and behaviour of

stainless-steel mass standards; 4, the measurement of

the density of Uquids and solids; 5, force measurement;

6, 7, 8 and 9, the measurement of high, medium, low

and very low pressures, respectively. In many of these fields, in addition to the individual and collaborative projects under way, international

comparisons had taken place or were in progress and

reports were presented. The CCM also discussed

recent advances in high-accuracy weighing and bal- ance design and the third verification of national

prototypes of the kilogram now in its preliminary

stages at the BIPM. It is expected that the first part of

this operation, the comparison of the international

prototype with its copies at the BIPM, will have been

completed by the end of 1988. Over the next three years or so, national prototypes will come to the BIPM in groups for verification. Since the last time

such a large scale verification took place, some forty years ago, the accuracy of the best balances has improved by at least a factor of ten. The present verifi- cation is being carried out using the NBS-2 balance

which is a single-pan, knife-edge balance that has an

accuracy of about 1 |j,g in the comparison of Pt/Ir mass

standards, i. e. a relative uncertainty at the level of one

standard deviation of about 1 part in 10^.

Comite Consultatif pour les Etalons de Mesure

des Rayonnements lonisants

Section I (X-rays, y-rays and electrons) and Section III (Neutrons) of the Comite Consultatif pour

les Etalons de Mesure de Rayonnements lonisants

(CCEMRI) met from 8th to 11th and 18th to 20th of

April respectively. During the meeting of Section I, the work of the BIPM was reviewed and the results of recent comparisons of national standards were dis- cussed. The need for comparisons of absorbed-dose

standards for radiation of energy above 1 MeV was

stressed. Two Recommendations were adopted and

sent to the CCEMRI for approval at its next meeting.

At the meeting of Section III, the BIPM work on

neutron measurements was reviewed. The published

results of completed neutron-emission-rate and flu- ence-rate comparisons, the NPL neutron-dosimetry

comparison and the preliminary report on the BIPM

neutron-dosimetry comparison were all discussed.

Work in the BIPM Laboratories

Among the principal activities in the laboratories

of the BIPM over the past year have been the following:

- Electricity section : a limited comparison of 1 Q

national reference standards of resistance was carried out to check the consistency among the various real- izations of the quantum Hall effect now being made in

1 1 national laboratories and the BIPM. Expressed in terms of the BIPM's representation of the ohm,

^69-Bi> on the central date of the comparison, the

measured results for the quantized Hall resistance in

five of the six laboratories claiming uncertainties of

4 parts in 10® or less are within a range of 7 parts in

10®. The overall spread of all results was 6 parts in 10'. Refinements in the BIPM quantum-Hall-effect system,

based upon a cryogenic current comparator, now al- low C2e9-Bi to be monitored with an uncertainty of 1.5 parts in 10®. Using Josephson junctions made from the new

high-Tc superconducting material a measurement of

2 e/h was made having an uncertainty of 3 parts in 10^

and which differed from the value obtained from a

metallic Josephson junction by about 5 parts in 10^.

- Mass section : as already mentioned, work began on the third international verification of national prototypes of the kilogram. The BIPM flexure-strip bal- ance has been used to carry out an experiment to search for a nuclear-isospin-coupled "fifth-force". No

evidence for the existence of such a force was found,

but during the experiment weighings of 2.3 kg masses were carried out that showed remarkably small stan- dard deviations of about 5 parts in 10^^.

- Time section : as part of the regular analysis of

clock comparisons made using the GPS satellites which go to make up International Atomic Time (TAI)

and Universal Coordinated Time (UTC), it was found

that for laboratories within about 1000 km of each

other errors in coordinates could be discerned and

corrected for with an accuracy of about 1 5 cm in the geocentric coordinates x,y and z. As a result, time

comparisons between these (European) laboratories can now be made to within a few nanoseconds.

- Laser section : important international comparisons of laser wavelengths and frequencies have been

carried out between a total of 8 countries, mostly at wavelengths in the visible (A = 633 mm) but also one in the infrared {A = 3.39 ^lm). Uncertainties in these comparisons are at level of about 1 part in 10^ * and differ- ences between lasers from participating countries were

found, in many cases, to be hardly greater than this.
- Ionizing radiations section : participation in the

international comparison of the measurements of activity of iodine-125 has begun. Two methods of spec- trum analysis are being used in order to understand

better the results obtained earlier during a limited

comparison of measurement of activity of the same

nuclide. The long-term work on counting methods

and counting statistics continues with much emphasis

now being placed on the method of 'generalized dead

times'. - Radiometry: new work has begun in radiometry.

Equipment has been built and is now in operation for the comparison of calibrations of silicon diodes and

thermopiles. The method of self-calibration of silicon diodes has been used and experience is being gained so

that international comparisons and, in due course,

calibrations can be carried out at the BIPM.

Other News

During the meeting of the CIPM in October 1988

a new building (of some 900 m^ in surface) was inaugurated. It will house a library and offices for scientific

staff, the secretariat and the Director. This completes

the second stage of a long-term plan for buildings at the BIPM, the first stage of which was a new laboratory building for laser work opened in 1984. On 31st July 1988 Dr. P. Giacomo, who had been

Director of the BIPM since 1978, retired and was suc- ceeded by Dr. T.J. Quinn, previously Deputy Director.

Publications

Since October 1987 the BIPM has published:

18^ Conference Generale des Poids et Mesures

(1987), Comptes Rendus, 108 pages. Proces-verbaux des seances du Comite International

des Poids et Mesures, tome 55 (76^ session, octobre

1987), 178 pages. Comite Consultatif de Photometrie et Radiometric, W session (1986), 183 pages. Comite Consultatif de Thermometrie, 16^ session

(1987), 162 pages. Rapport Annuel du BIHpour 1987 (about 72 pages

covering the contribution of the BIPM on time scales). Circulaire D du BIH (monthly) (contribution of

the BIPM on time scales). Circulaire T (monthly), circular of the BIPM

taking over the part "Temps" of the circular D of the BIH (Circular Tl for January 1988 was published on March 1st, 1988).

Le BIPM et la Convention du Metre, illustrated brochure in French and in English, 47 pages, widely

circulated through various national laboratories.

One should also mention some 40 articles published by staff members in scientific journals and a dozen BIPM reports; the complete hst of these pubHcations may be found in the Proces-Verbaux du

CIPM, 77th meeting (1988), which will appear in 1989. Copies of these documents may be obtained upon

application to: Mr le Directeur du BIPM, Pavilion de

Breteuil, F-92312 Sevres Cedex, or from the Librairie

Offilib, 48 rue Gay-Lussac, F-75240 Paris Cedex 05.
Journal of Research of the National Bureau of Standards

Special Report on Electrical Standards

Report on the 17^^ Session

Ofthe Consuhative Committee

On Electricity

Volume 92 Number 1 January-February 1987

B. N. Taylor

National Bureau of Standards

Gaithersburg, MD 20899

This report provides the background for and summarizes the main results of the 17th session of the Consultative Committee on Electricity (CCE) of the In- ternational Committee of Weights and Measures held in September 1986. In- cluded are decisions made by the CCE

which promise to have a profound ef- fect on the standardization of national representations of the volt and ohm and

thus on the international compatibility of electrical measurements. In particular, on January 1, 1990, worldwide changes

in the basis for such representations are planned which will lead to an increase

in the U.S. legal unit of voltage of about

9 parts-per-million (ppm) and in the U.S.

legal unit of resistance of about 1.5 ppm.

Key words: Consultative Committee on

Electricity; electrical units; Josephson

effect; ohm; quantum Hall effect; volt.

Accepted: December 11, 1986

1. Background

The 17th session of the Consultative Committee

on Electricity (CCE) of the International Committee of Weights and Measures (CIPM) met September 16-18, 1986, at the International Bureau of

Weights and Measures (BIPM), which is located in Sfevres (a suburb of Paris), France. NBS Director

Ernest Ambler, who was recently elected President of the CCE by the CIPM, chaired the meeting

and the author attended as NBS representative.

The CCE is one of eight CIPM Consultative

Committees which together cover most of the ar- eas of basic metrology [1]^ (see fig. 1). These committees, which may form temporary or permanent

'Working Groups' to study special subjects, coordi- nate the international work carried out in their re- spective fields, assist the CIPM in supervising the

About the Authon B. N. Taylor is a physicist and

Chief of the Electricity Division in the Center for Basic Standards. The Center is within the NBS

National Measurement Laboratory.

'Numbers in brackets indicate literature references.

work of the BIPM in these fields, and propose rec- ommendations concerning amendments to be made

to the definitions and values of units. The CIPM

acts directly on these recommendations or submits

them for approval to the General Conference of

Weights and Measures (CGPM) if they will have a broad impact.

The BIPM, whose principal task is to ensure worldwide uniformity of physical measurements, was established by the Treaty of the Meter signedin

Paris on May 20, 1875. (The number of signatories,

originally 17, has now grown to almost 50.) The

BIPM is supervised by the CIPM which in turn is under the authority of the CGPM. The CGPM

consists of delegates from all of the member countries of the Treaty of the Meter and presently meets

every four years. The CIPM consists of 18 members each representing a different country and

presently meets every year. In general, a consultative committee meets every few years, has as its president a member of the CIPM, and is composed

of delegates from the major national standards
Journal of Research of the National Bureau of Standards

TREATY OF THE METER

(20 May 1875)

Diplomatic

Established CGPM and BIPM

CGPM

Technical

Promotes SI Elects CIPM

Approves major CIPM decisions

Allocates funds to BIPM

v////y////////y/y/////////y////////y////y/////////y////y///////////. '///////^y///y/////////y//y///////////y///y/yy/y//////y////////////,,///y/y//////y/^/^/yy/y/////j CIPM

^^nn/^^

Sets up Consultative Committees

Makes proposals to CGPM

Directs operation of BIPM CONSULTATIVE

' ' '' .COMMITTEES« - ^

[

I ^0^x4

Radiometry

\ Temperature

\ Meter Second

Ionizing Radiation Units

\^ Mass

Assures worldwide uniformity

of physical measurements

Maintain and disseminate national standards Cooperate with BIPM and provide members for Consultative Committees

Figure 1-Schematic depiction of how basic measurement imits and standards are coordinated

internationally. (Treaty of the Meter: La Convention du Mitre; CGPM: Confirence Giniral des Pdds et Mesures or General Conference of Weights and

Measures; CEPM: Comiti Inter- national des Poids et Mesures or

International Committee of Weights and Measures; BIPM:

Bureau International des Poids et Mesures or International Bureau

of Weights and Measures.)

CDQ.

05 tr

(0Q.

1

-1

-2

-3

-4

-5

-6

-7

-8

-9

•10

SI VOLT (V) ppm

+4.50 ppm

V^\V\\\Vv.\VVNS.S.XVS.XV\\.S\V +1.32 ppm

U.S.S.R.

FRANCE

-3.3 ppm

-6.5 ppm

-1.20 ppm

ALL OTHER ^ „ COUNTRIES -^-S ppm

U.S. -9.0 ppm

Figure 2-Graphical comparison of the representation of the SI

volt of various countries based on the Josephson effect, Vlab> and of Vlab with the SI volt, V. The value of Vlab indicated by 'All Other Countries' is based on the value of the Joseph- son frequency-voltage ratio 2e/h recommended by the CCE

of the CIPM in 1972: (2e/A)ccB-72 = 483594.0 GHz/V. (See

footnote 2 for those coimtries which use this value.)

laboratories as well as from specialized institutes,

and individual members appointed by the CIPM.

The focus of the 17th session of the CCE, which was attended by some 30 individuals from 15 countries, was the use of the Josephson and quantum

Hall effects to define and maintain national or laboratory representations of the units of electrical potential difference and resistance of Le Systime

International d'Unites or International System of

Units (abbreviated SI): the SI volt (V) and SI ohm

(H), respectively [2]. (The national representations are usually designated by the symbol Vlab aiid Hlab where LAB stands for the acronym of the

national standards laboratory of the country in question, e.g., NBS.) More specifically, at its 16th

session in March 1983 the CCE had concluded that

[3] (see fig. 2):

(i) The value 483594.0 GHz/V for the Joseph- son frequency-voltage ratio 2elh (e is the el- ementary charge and h is the Planck

constant) which it had recormnended at its 13th session in 1972 [4-7] for defining and

maintaining national representations of the

SI volt is significantly in error. (Current evi- dence indicates that (2efh)ccEr-72 is about

eight parts-per-million or 8 ppm smaller

than the SI value of 2e/h and thus Vlab is about 8 ppm smaller than V for those national laboratories which use the CCE value

[2,8].)

(ii) It was highly likely that the recently discovered quantum Hall effect would soon be developed to the point that the quantized Hall

resistance Rh = A/e^ = 25812.8 CI could be

used to define and maintain national units of

resistance consistent with the SI ohm to
Journal of Research of the National Bureau of Standards

within a few tenths of a ppm. (More recent work indicates that a few hundredths of a ppm is quite feasible [9].) The CCE was also aware of the following:

(iii) Four different values of 2« /A are in use in the national laboratories [2]. [The U.S.,

France, and the U.S.S.R. use values of lelh

which differ by -1.20 ppm, +1.32 ppm,

and +4.50 ppm, respectively from

(2e/h)ccR.n and hence the national voltage

units of these countries differ by these

amounts from the national units of those

countries which use (2e//i)ccE-72-^

(iv) The various national units of resistance,

most of which are based on the mean resis- tance of a group of precision wire-wound

resistors, differ from each other and the SI

unit by up to several ppm and some are

drifting in excess of 0.05 ppm per year. (Current evidence indicates that on January

1, 1986, the various Olab were from 0.2 ppm

larger to 3.3 ppm smaller than Cl and

d0.ij^ldt lies in the range -0.07 to +0.07

ppm/year [8].) As a consequence of (i) through (iv), the CCE

decided at its 16th session in 1983 to hold its 17th

session in 1986 in order to consider the possibility

of recommending for adoption a new value for 2elh, consistent with the SI, to be used by every

laboratory which employs the Josephson effect to

define and maintain its representation of the SI

volt; and a value of Rn, consistent with the SI, to be used by every laboratory which chooses to employ the quantum Hall effect to define and maintain

its representation of the SI ohm.^

Considerable preparatory work was carried out

during the two years prior to the CCE's 17th session:

• Immediately after the formal close of the 1984

Conference on Precision Electromagnetic

Measurements (CPEM 84, held August 20-24,

1984, in Delft, The Netherlands), active re- search workers from the national standards

laboratories and other interested parties met

informally to discuss values of lelh and i?H- Many new and relevant results were also pre- sented at the conference itself [10].

• In the U.S., NBS Director Ambler sent a letter in early 1985 to over 30 U.S. organizations,

^ These include Australia, Canada, the Federal Republic of Germany, Finland, the German Democratic Republic, Italy, Japan, The Netherlands, and the U.K., as well as ±e BIPM.

^ Basing Hlab on Rn would, of course, eliminate the drift in IIlab (i-e., make dilu^ldt = 0).

companies, and individuals representing industry, government, science, and academia and

having an interest in basic electrical measure- ments and standards at the highest levels of

accuracy. The purpose of the letter was to give the U.S. scientific and technological communities the opportunity, well in advance of

the 1986 CCE meeting, to provide NBS with

advice and guidance on the subject of changing the U.S. electrical units. (The U.S. Legal

Volt Vnbs would increase by about 9 ppm and

the U.S. Legal Ohm Hnbs by about 1.5 ppm if the CCE were to recommend a new value for lelh consistent with the SI and a value of i?H

also consistent with the SI.) The comments re- ceived were presented during CPEM 86 (held

at NBS Gaithersburg, June 23-27, 1986) at a

special session with active audience participation entitled "Changes in the Electrical Units." In addition to the U.S. presenter, viewpoints

from other countries were given by speakers

from the Federal Republic of Germany,

Japan, and the U.K.* The three key points

which emerged from this session were:

(a) Changing national voltage and resis- tance units will outweigh the considerable costs of making the changes if and

only if complete international uniformity

of all the national units of the industrial- ized countries is achieved.

(b) The changes must be well justified by

the data; all of the available information

must be analyzed by knowledgeable ex- perts and no changes made unless the

uncertainties in the values of lelh and

i?H are sufficiently small that it is highly

unlikely that further changes will be

necessary in the forseeable future.

(c) At least one year should be allowed

from the date of the official announce- ment that the changes will take place to the date of their actual implementation

so that industry will have sufficient time

to prepare itself properly.

Many researchers took the occasion of CPEM

86 to present their latest results on values of

lelh and R^ [11]. There was an informal meeting of active re- search workers and other interested parties to

discuss values of lelh and i?H just after the

close of CPEM 86 as there was at the close of

CPEM 84.

* Written versions of the talks given during the session will appear in Ref. [11].
Journal of Research of the National Bureau of Standards

• Fifty-nine dociunents were submitted to the

CCE by the CCE members (i.e., by the national standards laboratories of the member

countries) in support of the CCE's deliberations. These docimients included new results as well as comments on the key issues facing

the CCE concerning the adoption of a new

value for 2elh and a value of Ra.

• A detailed provisionary agenda containing some 25 items was prepared for the 1986 CCE

meeting with extensive supporting material in succinct, summary form [12].

2. CCE 17th Session Discussions and

Principal Decisions

2.1 Josei^son Effect

The principal Josephson effect topics reviewed

in detail by the CCE were [12] (i) the value of 2elh

used by each national standards laboratory to maintain its representation of the SI volt and the

accuracy achieved; (ii) the observed agreement

among national voltage standards based on the

Josephson effect; (iii) the uncertainties associated

with intercomparing these voltage standards using

transportable standard cells and 2^ner diode

devices; (iv) the values of 2elh in SI units and their uncertainties obtained by direct force balance measurements and indirectly from fundamental constant determinations; (v) the prospects for future SI

values of lelh with their expected uncertainties

and dates of availability; and (vi) the need for fur- ther intercomparisons of national voltage standards

and Josephson apparatus.

With regard to (i), the uncertainties achieved were noted to be generally in the range 0.01 to 0.1 ppm*, although Josephson arrays may enable imcertainties smaller than 0.01 ppm to be achieved

routinely [13]. Under (ii) and (iii), it was concluded

that the agreement between national Josephson

voltage standards was generally better than 0.1 ppm but that it was difficult to demonstrate this level of consistency using volt transfer standards.

With regard to (iv) and (v), the CCE decided that although the data currently available (both direct

and indirect) could provide a value of 2elh in SI

units with an uncertainty of about 0.2 ppm, additional data expected to be available within two

years would significantly increase confidence in the reliability of the value. Finally, under (vi), the

CCE concluded that a formal, broadly based, inter- national comparison of national units of voltage

would not be useful because of the unreliability of

^ Throughout this paper, all uncertainties are one standard

deviation estimates.

volt transfer standards but that those laboratories

involved in determinations of 2elh should attempt

to compare their units using the best transfer stan- dards at their disposal.

The result of the CCE's review of these Joseph- son effect topics was the following formal declaration:

Declaration E 1 (1986)

Concerning the Josephson effect for maintaining

the representation of the volt. The Comite Consultatif d'ElectricUe

recognizes - that as an organ of the Convention du Mfetre one of its responsibilities is to ensure the propagation and improvement of the SI, the unit system in use throughout the world, - that worldwide uniformity and constancy over a long period of time of national representations of the volt are of great technical and eco- nomic importance to commerce and industry, - that many national standards laboratories use

the Josephson effect to maintain a highly stable representation of the volt but that not all use the same value for the quotient frequency to voltage, - that the value of this quotient (483594.0 GHz/

V) declared by the CCE in 1972 and which most

national laboratories use to maintain representations of the volt is now known to be in error by a

significant amount, - that various laboratories have carried out di- rect realizations of the volt or determinations of

fundamental constants which can yield an indirect

value of 2e//i in SI units, - that other national laboratories expect shortly

to complete similar realizations or determinations,

is of the opinion - that the value of the quotient frequency to voltage used to maintain a realization of the volt by means of the Josephson effect must be consistent

with the SI, - that a new value, more consistent with SI, can soon be adopted for use by all laboratories, - that this new value should be adopted simultaneously by all countries concerned,

in consequence, the CCE

- establishes a Working Group charged with

making a proposal to the CCE for a new value to be based upon all relevant data that become avail- able before June 15, 1988, - decides to meet in September 1988 with a view

to recommending the new value of this quotient to come into effect on January 1, 1990,
Journal of Research of the National Bureau of Standards

- gives notice that the new value is likely to be

liigher than the present one by about 8 parts in 10*, furthermore, the CCE

- recommends that national laboratories vigorously pursue their work on the realizations of the

volt, the intercomparison of these realizations, and

the determination of the constants in question and

communicate without delay all their results to the

Working Group, - recommends that laboratories do not change

their value for this quotient until the new value comes into effect, - believes that the value to be adopted will be

sufficiently accurate, in terms of SI, that no further

change will be required in the foreseeable future. The unsatisfactory state of volt transfer stan- dards led the CCE also to develop Recommendation E 1 (1986), the thrust of which is that the

national standards laboratories "actively pursue

the study and improvement of transportable stan- dards with which the volt may be transferred from

one laboratory to another ..." Both Declaration E

1 (1986) and Recommendation E 1 (1986) were

subsequently approved by the CIPM at its October

1986 meeting [14].

2.2 Quantum HaU Effect

The principal quantum Hall effect (QHE) topics

reviewed in detail by the CCE were [12] (i) the

values and accuracies achieved in measurements of

the quantized Hall resistance Rh = h/e^in terms of

national representations of the ohm; (ii) the values

of i?H in SI units and their uncertainties obtained by

direct calculable capacitor-based measurements

and indirectly from fundamental constant deter- minations; (iii) the prospects for future SI values of

Rh with their expected uncertainties and dates of

availability; (iv) the results of recent comparisons

of national units of resistance using transportable

resistance standards; (v) the agreement among the

present values of Ra in laboratory units and the

agreement between various realizations of the SI

ohm based on the calculable capacitor; (vi) the precautions required to ensure reliable results from a

quantized Hall resistance sample and the availability of good samples; and (vii) the need for further

intercomparisons of national units of resistance. With regard to (i), most laboratories were able to determine Rh in terms of their national ohm with an uncertainty in the range 0.02 to 0.1 ppm. Under

(ii) and (iii), the uncertainties of the values of Rh in SI units, both direct and indirect, varied between

0.020 to 0.32 ppm, and most values agreed with the

value having the smallest uncertainty within 0.2

ppm. A number of new and possibly more accurate

results could be expected within two years. With

regard to (iv), a 0.05 ppm transfer uncertainty or even better can apparently be achieved if the trans- port resistors are carefully selected and used.

Under (v), the CCE concluded that most measure- ments of Rh in laboratory units agreed within an

imcertainty of 0.2 ppm but that the agreement

among realizations of the SI ohm was somewhat

worse. With regard to (vi), the CCE decided that while tests were available which could be used to ensure reliable results from a particular quantized

Hall resistance sample, and that a number of good

samples are already in hand, increased understanding of the QHE as well as additional metrologically

useful samples were highly desirable. Finally, un- der (vii), the CCE decided that it would be useful to conduct an international comparison of l-H re- sistance standards to facilitate the comparison of

measurements of Rh in laboratory units. The result of the CCE's review of these quantum

Hall effect topics was the following formal declaration: Declaration E 2 (1986)

Concerning the quantum Hall effect for maintaining a representation of the ohm.

The Comite Consultatif d'Electridti

recognizes - that as an organ of the Convention du Mfetre one of its responsibilities is to ensure the propagation and improvement of the SI, the unit system in use throughout the world, - that worldwide uniformity and constancy over

a long period of time of national representations of

the ohm are of great technical and economic importance to commerce and industry, - that the application of the quantum Hall effect as a means of maintaining a stable representation of

the ohm is being developed rapidly in many national standards laboratories, - that the quantum Hall effect is providing very

reproducible results from one laboratory to another, but that the number of usable samples avail- able is insufficient for present needs, - that experience is leading to tests that provide

assurance of both reproducible and accurate results from a selected sample, - that no laboratory has yet adopted a value of

the quantized Hall resistance i?H to maintain its lab- oratory representation of the ohm, - that various laboratories have determined Rh

in SI units using both the calculable capacitor and

determinations of fundamental constants, - that additional results for Rh in SI units are
Journal of Research of the National Bureau of Standards

expected to become available in the near future,

is of the ophiion - that the same value of Rh should be adopted

simultaneously by all those laboratories that decide

to use the quantized Hall resistance as their representation of the ohm, - that this value should be consistent with SI, - that such a value can soon be adopted,

in consequence, the CCE

- establishes a Working Group charged with

making a proposal to the CCE for a value of Ra to be based upon all relevant data that become avail- able up unitl June 15, 1988 and with developing

detailed guidelines for the proper use of the quantum Hall effect to maintain a representation of the

ohm,

- decides to meet in September 1988 with a view

to recommending the value of Rh to come into ef- fect on January 1, 1990, - gives notice that the adoption of this value for Rh may lead to a change in national and the BIPM

representations of the ohm; this change should in general not exceed 2 parts in 10*, furthermore, the CCE

- recommends that national laboratories - vigorously pursue their work to understand better the quantum Hall effect, - encourage the increased availability and

distribution of good quantum Hall effect samples, - determine the value of Rh in SI xmits both by the direct realization of the ohm

and the determination of appropriate

fundamental constants, - carry out bilateral comparisons as seem

appropriate, and communicate without

delay all their results to the Working

Group, - recommends that the BIPM organize during

1987/88 an international comparison of one ohm

resistance standards in connection with the quantum-Hall effect work, - recommends that no laboratory should adopt a

value of Rh for its representation of the ohm or use

the quantum Hall effect to alter the present drift rate until the recommended value comes into ef- fect, - believes that the value to be recommended in 1988 will be sufficiently accurate, in terms of SI, for no change to be required in the foreseeable future. The less than satisfactory current state of understanding of the QHE and the limited availability of

good samples led the CCE also to develop Recommendation E 2 (1986) which encourages (a) studies

of QHE sample manufacture and characterization,

(b) the provision of an adequate supply of high

quality QHE devices for metrological purposes by

industry and research laboratories, (c) better theoretical and experimental understanding of the

QHE, and (d) comparisons of QHE devices under

the auspices of the BIPM. Both Declaration E 2

(1986) and Recommendation E 2 (1986) were also subsequently approved by the CIPM at its October

1986 meeting [14].

3. Conclusion

If all proceeds as planned, that is, if the several new values for 2e/h and Rh in SI units which are expected to become available by June 15, 1988,

agree with earlier results within acceptable limits,

then the CCE at its 18th Session in September 1988

will officially recommend for adoption a new value

for the Josephson frequency-voltage ratio 2e/h

consistent with the SI, and a value of the quantized

Hall resistance Rh = h/e^ also consistent with the

SI, to be used by all national standards laboratories

and the BIPM to define and maintain their representations of the volt and ohm. These new values,

which would be implemented simultaneously

throughout the world starting January 1, 1990, are

anticipated to have an uncertainty of between 0.1 and 0.3 ppm. Moreover, the uncertainty associated

with using the Josephson and quantum Hall effects to define and maintain representations of the volt

and ohm should generally be in the range 0.01 to

0.1 ppm. As a consequence, starting January 1, 1990, the practical electrical units for voltage, resis- tance, and current of most industrialized countries

will be equivalent within an uncertainty no greater

than about 0.1 ppm and these units will be consistent with their respective SI units within an uncertainty no greater than about 0.3 ppm. Although

implementing these new representations will re- quire adjusting a large industrial inventory of stan- dards and instruments by significant amounts (e.g., in the United States the U.S. Legal Volt will in- crease about 9 ppm and the U.S. Legal Ohm about

1.5 ppm), the benefits of international uniformity of

electrical measurements and their consistency with

the SI which will result from the unit changes

should completely outweigh the costs of making

them.

References

[1] Com. Intl. Poids Mes. Com. Consult d'Electricitfi, Trav.

16* Session, 1983 (Bur. Intl. Poids Mes., Sfevres, France,

1983), p. E 105.
Journal of Research of the National Bureau of Standards

[2] Taylor, B. N., J. Res. Nati. Bur. Stand. 91 299 (1986).

[3] Ret [1], pp. E 3, E 9, E 111, E 116; and Giacomo, P., Metrologia20 25(1984).

[4] Com, Intl. Poids. Mes. Com. Consult d'Electricltd, Trav.

13« Session, 1972 (Bur. Intl. Poids Mes., Sfevres, France,

1972), p. E 13; Terrien, J., Metrologia 9 40 (1973).

[5] P. V. Stances Com. Intl. Poids Mes., 61* Session, 40 (Bur.

Intl. Poids Mes., Sfcvres, France, 1972), pp. 22, 100.

[6] Com. Intl. Poids Mes. Com. Consult d'Electricit6, Trav. U* Session, 1975 (Bur. Intl. Poids Mes., Sivres, France,

1975), p. E 6; Terrien, J., Metrologia 11 179 (1975).

[7] P. V. Stences Com. Intl. Poids Mes., 64* Session, 43 (Bur.

Intl. Poids Mes., Sivres, France, 1975), p. E 7.

[8] Taylor, B. N., and T. J. Witt, Document CCE/86-^ sub- mitted to the 16* Session of the Comit6 Consultatif

d'Electricitt, CIPM, 1986 (to be published in Ret [12]).

[9] Delahaye, F.; D. Dominguez, F. Alexandre, J. P. Andre, J. P. ffirte, and M. Razeghi, Metrologia 22 103 (1986).

[10] See the relevant papers in IEEE Trans. Instnun. Meas. IM-34 (June 1985).

[11] IEEE Trans. Instrum. Meas. IM-36 (June 1987, to be published).

[12] Com. Intl. Poids Mes. Com. Consult d'Hectricitd, Trav.

IT* Session, 1986 (Bur. Intl. Poids Mes., Sfcvres, France, to be published).

[13] Hamilton, C. A.; R. L. Kautz, R. L. Steiner, and F. L. Lloyd, IEEE Device Lett EDLr^ 623 (1985); Niemeyer,

J.; L. Grimm, W. Meir, J. R Hinken, and E. Vollmer,

Appl. Phys. Lett 47 1222 (1985).

[14] P. V. seances Com. Intl. Poids Mes., 75* Session, 54 (Bur.

Intl. Poids Mes., S^res, France, 1986, to be published).

IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT. VOL. IM-36, NO. 2. JUNE 1987 History of the Present Value of 2e Ih Commonly

Used for Defining National Units of Voltage and

Possible Changes in National Units of Voltage and

Resistance

BARRY N. TAYLOR, fellow, ieee

Abstract—The national standards laboratories of most major industrialized countries employ the Josephson eifect to define and maintain

their national or laboratory unit of voltage Vlab- The value of the Jo- sephson frequency-voltage ratio commonly used for this purpose, 2e/h = 483 594 GHz/Vlab» 's now known to be about 8 ppm less than the absolute or International System of Units (SI) value. Consequently, the

different national units of voltage are smaller than the SI unit by the same amount. One of the purposes of this paper is to review how this value o{2e/h was selected and, hence, the origin of the present inconsistency between national voltage units and the SI unit. The motivation

for such an historical study is the hope that it can benefit the selection of a new, more accurate value of 2e/h planned for the near future. Also discussed is the status of national units of resistance and the effect of defining and maintaining such units using a value of the quantized

Hall resistance consistent with the SI, as may be suggested in the near

future as well.

I. Introduction

THE SYSTEM of units in general use to express the

results of physical measurements is the International

System of Units (SI). Because the SI definitions of the

volt (V) and ohm (12) are difficult to realize with high

accuracy, national standards laboratories such as the U.S.

National Bureau of Standards (NBS) have historically used

practical representations of them to serve as the national or legal electrical units. For example, the mean EMF of

a particular group of electrochemical standard cells of the Weston type (each with an EMF of order 1.018 V) has

traditionally been used to define a laboratory or as-maintained national unit of voltage Vlab' and the mean resis- tance of a particular group of precision wire-wound resis- tors of the Thomas or similar type ( each with a resistance

of order 1 Q ) has similarly been used to define a laboratory or as-maintained national unit of resistance Alab- The

national unit of current Alab is then defined in terms of

^LAB and ^LAB by means of Ohm's law, Alab =

Vlab/^lab. and does not require its own separate representation.

The national standards laboratories of most major in- dustrialized countries now use the Josephson effect [1] to define their unit of voltage and maintain it constant in

Manuscript received June 23, 1987. The author is with the Electricity Division, National Bureau of Standards, Gaithersburg, MD 20899. IEEE Log Number 8613596.

time. A low-tempetaiure solid-state physics phenomenon,

the Josephson eff'ect occurs when two superconductors

separated by 1-2 nm (achievable with an oxide layer) are cooled below their transition temperatures. If such a Jo- sephson junction is exposed to microwave radiation of

frequency /, current steps appear in its current-voltage curve at quantized values of voltage. The voltage V„ of

the nth step and the frequency /are related by 2eV„ = nhf

where e is the elementary charge and h is the Planck constant. A Josephson junction can thus be viewed as a perfect frequency-to-voltage converter with the constant of

proportionality being the invariant fundamental-constant

ratio 2e/h. Because frequencies can be readily measured

to very high accuracy, the Josephson effect can be used

to define and maintain Vlab to an accuracy limited only

by the uncertainty with which the voltage across the Jo- sephson device can be compared with the 1.018-V EMF

of a standard cell. Typically this is in the range 0.01-0.1

ppm.' The standard cell now serves only as a "flywheel," that is, as a means of preserving or storing Vlab

between Josephson effect measurements.

II. Origin of CCE Recommended Value of 2e/h

The value oile/h in widest use for volt-maintenance

purposes

(2€//2)„„p ^ 483 594.000 GHz/Vlab (1)

is based on a 1972 recommendation of the Consultative

Committee on Electricity (CCE) of the International

Committee of Weights and Measures (CIPM). The rec- ommendation, officially known as Statement E-72, was

developed at the thirteenth session of the CCE held in October 1972 [2], [3]. Subsequently approved by the

CIPM at its sixty-first session [4] (also held in October

1972), and reiterated in 1975 by both the CCE [5], [6] and CIPM [7], it reads:

Statement E-72

The Consultative Committee on Electricity

Considering

that the Josephson effect enables potential steps to be re- produced with a high precision,

'Throughout, all uncertainties are one standard deviation estimates unless otherwise noted.

U.S. Government work not protected by U.S. copyright
IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL, IM-36, NO. 2, JUNE 1987

that several laboratories are using this device in order to maintain their realizations of the volt at a constant value; and

that their results make it possible to relate the value of the

potential steps of the Josephson effect to the realizations of

the volt maintained in several laboratories,

Considers from these results that, on January 1, 1969,

^69-81 was equal within half a part per million to the potential step which would be produced by a Josephson junction ir- radiated at a frequency of 483 594.0 GHz.

Here, Vgg.gi is the as-maintained or laboratory unit of

voltage of the International Bureau of Weights and Measures (BIPM) defined starting January 1, 1969 in terms of

a group of standard cells. ^ Since the EMF's of chemical

cells vary with time, V^q.bi is expected to be a time-dependent unit.^ Whether the 0.5-ppm uncertainty of State- ment E-72 was meant to correspond to a one-, two-, or

three-standard deviation estimate, or something else, was

never explicitly indicated by the CCE. The countries that use the CCE value of 2e/h include Australia, Canada,

Germany, Finland, Italy, Japan, the Netherlands, and the

U.K.

The value oile/h in terms of Vgg.BiC January 1, 1969)

given in Statement E-72 was derived by J. J. Denton [10]

of the National Physical Laboratory (NFL), U.K. Denton's analysis was based on a linear extrapolation oile/h

measurements carried out in Australia, Germany, the

U.K., and the U.S. from 1969 to 1972. The measure- ments were made in terms of the time-dependent laboratory units of these respective countries as defined by means of electrochemical cells and the extrapolated val- ues were converted to values expressed in terms of

V69.Bi( January 1, 1969). This conversion was based on

the differences Vlab( January 1, 1969) - V69.B1 calculated from the results of the 1967 and 1970 triennial in- ternational comparisons of national units of voltage (and

resistance) carried out using transportable cells (and resistors) at the BIPM.

Denton's calculation and thus CCE Statement E-72 was

subsequently confirmed by a more rigorous but similar

analysis based on linear least squares fits carried out by

E. R. Cohen and the author in conjunction with the 1973

least squares adjustment of the fundamental constants

[11]. The analysis, which included additional 2e/h measurements and volt unit intercomparison data which had

subsequently become available, indicated that V69-Bi( January 1, 1969) corresponded to a Josephson fre- quency of 483 593.987(90) GHz (0.19 ppm). Cohen and

the author have recently extended this analysis in conjunction with the 1986 least squares adjustment [12]. Taking into account the added results of the 1973 triennial international comparison and 41 2e/h measurements car- ried out at BIPM in terms of V69.B1 over the period of

^The BIPM is located in a suburb of Paris, France and is the "interna- tional" standards laboratory. Established by the Treaty of the Meter signed

in Paris May 20, 1875, its principal function is to ensure worldwide uni- formity of physical measurements.

'it was therefore supplanted on January 1, 1976 by the time-independent

unit V76.B1 based on the Josephson effect and the CCE value of 2e/h [8],

19].

October 1974 to July 1976, we obtain 483 593.876(48)

GHz (0.10 ppm). One may thus conclude that Statement

E-72 is correct since the value it recommends is only 0.26

ppm larger than this last and, presumably, highly reliable value.

III. Consistency of {2e/h)ccE with the SI, Other

Values in Use, and Implications

The CCE based its Statement E-72 on V69.bi( January

1, 1969) because it believed that this unit was highly consistent with the SI unit. In other words, the CCE recom- mended value of 2e/h was expected to be very nearly

equal to the SI value and, hence, any laboratory voltage

unit based on it was expected to be highly consistent with

the SI. Unfortunately, this has not turned out to be the

case. As long ago as 1976 the author pointed out the like- lihood oi{2e/h )ccE being from 4 to 10 ppm smaller than

the SI value [13]. Since then, force balance and similar

direct realizations of the SI volt [14], [15] as well as determinations of relevant fundamental physical constants

[16] have advanced to the point that one can now state that the SI value of 2e/h exceeds that suggested by the

CCE by about 8 ppm with an uncertainty of less than 0.5

ppm. As a consequence, the national unit of voltage of

each country that employs the Josephson effect and

(2e/h)ccE is smaller than the SI unit by about 8 ppm.

The inconsistency between Vlab and V means that

electrical and mechanical measurements of force, energy,

and power will not yield the same results. Perhaps even more significant, three countries currently use values of

2e/h, which differ from (2e/h)ccE- The Central Laboratory of the Electrical Industries (LCIE), France, employs [17]

(2e//t),_ = 483 594.64 GHz/Vloe (2)

the U.S. NBS uses [17]

{2e/h)^ = 483 593.420 GHz/Vnbs (3)

and the All-Union Scientific Research Institute of Metrology (or Mendeleyev Institute of Metrology (IMM)),

USSR, employs [18]

(2^A)iMM ^ 483 596.176 GHz/Vimm- (4)

The French value was chosen to prevent a discontinuity

in the French volt when converting from standard cells to the Josephson effect as the basis for Vlcie in the early

1970's. The U.S. value was chosen for the same reason. The USSR value was selected in the late 1970's to make

VjMM more consistent with the SI unit and is based on an IMM analysis of the results of certain fundamental constants experiments then available.

The consequences of (2)-(4) are as follows: the French

volt is 1.32 ppm larger than the volt of those countries

which use (2e//i)ccE and about 6.7 ppm smaller than the SI volt; the U.S. volt is 1.20 ppm smaller than the volt of these same countries and about 9.2 ppm smaller than

the SI unit; and the USSR volt is 4.50 ppm larger than
TAYLOR: HISTORY OF THE PRF.SF.NT VAl.UF OF 2<'//i

the volt of those countries which use (2e /h )cce ^"d about

about 3.5 ppm smaller than the SI volt. The chaotic situation regarding national units of voltage

as outlined in this section had led the CCE to plan to meet

in September 1986 with the aim of reviewing all relevant

experiments and, if justified, adopting a new value of

le/h consistent with the SI to be used by every national

standards laboratory (and BIPM) that employs the Jo- sephson effect to define and maintain its laboratory unit of voltage [19], [20]. In view of this possibility, we ex- amine further the origin of the inconsistency between

(2e/h)ccE and the SI value in the hope that such an analysis can benefit the selection process.

IV. Origin OF V69_Bi( January 1, 1969)

Since it was shown in Section II that statement E-72 is correct, the discrepancy between (2e/h)ccE and the SI value must arise from the unit V^qbiC January 1, 1969).

This unit was created by adjusting the unit Vbipm downward by 11 ppm, where VgiPM is the (time-dependent)

BIPM as-maintained volt prior to January 1, 1969 based on a group of standard cells. In actual fact, since Vbipm

and Vgg.Bi were based on the same group of cells, their only difference is the 11 -ppm redefinition. Formally, on

January 1, 1969

1 V69-B. = 1 X (1 - 11 X 10-^)Vb,pm = 0.999 989 Vbipm.

(5)

The observant reader will note that the 8-ppm differ- ence discussed in Section II between (2e/h)si and

(2e/h)ccE implies that on January 1, 1969 Vbipm was 3 ppm larger than the SI volt. Thus if the BIPM volt had

not been redefined on January 1, 1969 the CCE would

have recommended a value of 2e/h, only 3 ppm larger

than (2e/h)si, rather than the current value which is 8 ppm smaller than {2e/h)si.

The 11-ppm adjustment of Vbipm on January 1, 1969 was intended to bring the BIPM volt into agreement with

the SI unit. The CCE authorized the adjustment through

Recommendation E- 1 developed at its twelfth session held

in October 1968 [21], [22]. Subsequently approved by the CIPM at its fifty-seventh session (also held October 1968)

[23], it reads: Recommendation E-1 The Consultative Committee on Electricity

Considering

that the resistance and electromotive force standards of

the BIPM have defined since January 1, 1948 the reference values flBiPM and ^bipm to which are referred similar values

defined by the standards of the national laboratories;

that the BIPM considers that the time has come to bring

Qbipm and Vgip^ into better agreement with the ohm and the

volt obtained by absolute determinations,

Recommends that the Bureau International should be authorized to use the following new reference values starting from

January 1, 1969:

The Consultative Committee on Electricity notes that the national laboratories consulted during the meeting are ready

to adjust the values of their standards at the same date or soon afterwards, and it established that these adjustments

would ensure a better uniformity of measurements throughout the world, together with a better agreement with the SI

definitions of the electrical units. As indicated in the last sentence of Recommendation

E-1, most national standards laboratories also adjusted

their national unit of voltage"* (resistance) on January 1

1969 in order to bring it into agreement with

V69_Bi( January 1, 1969) (n69_Bi( January 1, 1969)). The

size of the adjustment was determined by the difference

between Vlab and Vbipm ( ^lab and ^bipm ) obtained in the 1967 triennial international comparison of national units

of voltage (resistance). For example, on January 1, 1969

the NBS unit of voltage V^bs was decreased by 8.4 ppm.

This should be compared with the 9.2 ppm by which V^bs

as defined by (3) must now be increased to bring it into agreement with the SI volt.

It is important to recognize that Recommendation E-1

does not give the uncertainty to be associated with

V69-Bi( January 1, 1969) relative to the SI unit. That is, there is no indication of how well V69.bi( January 1, 1969)

represents the SI volt. Indeed, to take an extreme example, if the uncertainty to be assigned the 11-ppm correction were itself 1 1 ppm, it would be difficult to justify the change. It is, therefore, important to examine how the 11 ppm came about, and its uncertainty.

The 11-ppm correction was apparently first suggested

by J. Terrien, then BIPM Director, at the eleventh session

of the CCE held in October 1965 [25], [26]. The details of his analysis were given in a paper presented at the 1966

Conference on Precision Electromagnetic Measurements

and subsequently published in the conference proceedings

[27]. Based on the 1956 NBS current balance realization

of the ampere, a similar 1962-1963 NPL determination,

the results of the 1964 triennial international comparison

of national units of voltage and resistance, and five real- izations of the SI definition of the ohm carried out by five national laboratories from 1957 to 1964 using several dif- ferent methods, Terrien concluded that

1 nBIPM = 1.000 000 n (6)

with an uncertainty which might be about 1 or 2 /xQ; and

that

1 Vbipm = 1.000 011 V (7)

n,69-BI — ^BIPM = Vb,pm(1 - 11 X 10-^).

with an uncertainty of about 3 or 4 /a V. Although it was

not made clear just what the 3- or 4-/iV or ppm uncertainty associated with (7) meant, it was most likely a probable error (PE) or 50-percent confidence level esti- mate since this was the approach used in the NBS and

NPL ampere experiments [28]. Assuming normally dis- tributed data, the one standard deviation uncertainty es- timate would then be 4.5-6 ppm (i.e., 1 standard devia-

"it should be noted that prior to 1972, all national units of voltage were

based on electrochemical cells.
IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT. VOL. 1M-.16. NO. 2, JUNE 1987

tion = 1.48 X PE [28]). The values obtained from the two separate experiments were given as 1 VgiPM =

1.0 000 140 V and 1 Vb,pm = 1.0 000 084 V, respectively. In preparation for the October 1968 CCE meeting, a number of documents were submitted to the Committee

which bore on the question of the relationship between

^BiPM and fl, and Veip^ and V. In Document CCE/68-9,

the National Standards Laboratory of Australia (NSL),

now known as the National Measurement Laboratory

(NML) of the CSIRO Division of Applied Physics, re- ported additional results from their calculable-capacitor

realization of the SI ohm which showed that on February

12, 1967, the mean date of the 1967 triennial international

comparison of national units of voltage and resistance, 1 ^BiPM = 10 — (0.2 ± 0.7) /ifi. This result was generally

taken as confirmation of the equality of Qbipm and fl, implying that the difference between the BIPM ampere and

the SI ampere was essentially the same as the difference

between the BIPM volt and the SI volt. In Document CCE/68-11, NBS deduced the value

1 Vb,pm = 1 V + 11.0 mV (8)

based mainly on the following: 1) a slightly revised value

for the result of their 1956 current balance experiment (the

change was due to improved knowledge of the gravitational acceleration); 2) a 1967 SI ampere realization using an improved version of the NBS Pellat torque balance; 3) a slightly revised value for the result of the 1962-1963

NPL current balance experiment; and 4) a realization of

the ampere derived from the NBS and NPL determinations of the proton gyromagnetic ratio by the low-field

method and the Kharkov State Institute of Measures and

Measuring Instruments (USSR) determination of the same

quantity by the high-field method [28]. In Document CCE/68-34, IMM derived the result

BIPM = IV + \\.2 fiW (9)

in a manner essentially identical to that of NBS but also included low-field proton gyromagnetic ratio measure- ments from Japan and the USSR [28].

The 1 1-ppm correction to Vbipm first suggested in 1965

by Terrien was thus confirmed by the results and analyses

given in these three documents, and the basis for putting

forward Recommendation E-1, apparently firmly established. However, it must be emphasized that neither Document CCE/68-11 nor Document CCE/68-34 included any

estimate of the uncertainty to be associated with their deduced values (i.e., with (8) and (9)). This uncertainty

would have had to been in the 3-4-ppm range based on

the a priori uncertainties of the several experiments in- volved.^ One wonders whether the following critical question was ever asked: is the uncertainty in the pro-

'A rigorous treatment ol these and additional experiments carried out by

the author and his coworkers in 1969 within the framework of a least squares adjustment of the fundamental constants |28] yielded I Vf,,.,,, = 1 x [ 1 —

(11.4 ± 2.6) X 10 "] V„„.M en January 1, 1969.

posed 1 1-ppm correction to Vgip^ sufficiently small to justify making it? Perhaps if greater attention had been

paid in these two documents to calculating the uncertainty

to be associated with the correction, the CCE would have

recognized that the latter was only three to four times

larger than the former and concluded that even though the

various measurements agreed, in view of this small "margin of safety," the best approach would be to do nothing

until more accurate results were available. As it now

stands, the thousands of laboratory and industrial voltage

standards and related instruments throughout the world

which have a precision of say 25 ppm or better will no

doubt eventually have to be readjusted.

V. Laboratory Units of Resistance and the

Quantum Hall Effect

The mean resistance of a particular group of precision

wire-wound resistors is still the means by which most national standards laboratories define their laboratory unit

of resistance. However, starting in 1963 the NML in Australia has had in continuous operation a calculable capacitor-based apparatus which has been used to realize periodically the SI definition of the ohm and determine the

relationship between Qnml and fi [29]. Indeed, starting in 1969, the NML has defined their laboratory unit in terms of their realization of the SI ohm and maintained Onml

equal to Q with an uncertainty on the order of 0. 1 ppm.

Through the oflScial BIPM triennial international comparisons of national units of resistance using transportable

reference resistors and similar official comparisons in- volving a limited number of countries, it is possible to determine the drift rate and value relative to the SI of the

national unit of resistance of most industrialized countries

based on the NML calculable capacitor measurements.

For example, a recent analysis by E. R. Cohen and the

author in connection with the 1986 least squares adjustment of the constants [12] indicates that ^bipm is changing

at a rate of - (0.0 566 + 0.0 015 ) ppm /per year and that on January 1, 1985, 1 Qbipm = 1 fi - (1.46 + 0.14) /iQ.

For NBS the respective figures are - (0.0 589 + 0.0 065)

and -(1.23 + 0.15), while for the National Research

Council (NRC) of Canada, -(0.150 ± 0.060) and - (3.37 + 0.16). Similar values are obtained for other

laboratories.

Although a number of other countries now have, or

shortly will have, fully operational calculable capacitor SI ohm realization experiments, it still remains a complex

undertaking. (The fact that only one laboratory in the

world has had such an apparatus in continuous operation

since the method was developed in the early 1960's attests to its difficulty.) Thus metrologists heartily welcomed the

discovery in 1980 of the quantum Hall effect (QHE) since

the QHE promises to do for resistance-unit definition and

maintenance what the Josephson effect has done for volt- age-unit definition and maintenance [30], [31].

Like the Josephson effect, the QHE is a low-temperature solid-state physics phenomenon. However, the materials involved are semiconductors rather than supercon-
TAYLOR: HISTORY OF THR PRF.SENT VALUF, OF le/h

ductors. The QHE is characteristic of a two-dimensional

electron gas (2DEG) realized, for example, in classic Hall-bar geometry, high-mobility semiconductor devices

such as silicon MOSFET's and GaAs-Al^Gai _^As het- erostructures when the devices are placed in a magnetic

field of order 10 T and cooled to a few kelvin. Under these

conditions the 2DEG is completely quantized and there

are regions in the curve of Hall voltage versus gate volt- age for a MOSFET, or Hall voltage versus magnetic field for a heterostructure, where the Hall voltage remains constant as the gate voltage or magnetic field is varied.^

On these so-called Hall plateaus the Hall resistance

/?/y (/ ), defined as the ratio of the Hall voltage of the /th

plateau Vn{i) to the current / through the device, Rnii) = Vf^(i )//, is quantized and given by /?/y (/ ) = h/ie^i

with the quantum integer / equal to the plateau number.

Numerically, h/e^ ~ 25 812.8 Q and, hence, the resis- tance of the readily obtainable / = 4 plateau is = 6453.2

Q. A QHE device can thus be viewed as a resistor whose

resistance depends only on the fundamental-constant ratio h/e^. As such it can be used to define and maintain

^LAB to an accuracy limited only by the uncertainty with

which the resistance of the device (when on a plateau) can

be compared with the 1-Q resistance of a standard resistor. Eventually this is expected to be in the range 0.01-

0.1 ppm for all laboratories. In analogy with the standard

cell and the Josephson effect, the standard resistor would

serve only to store Qlab between QHE measurements.

The quantized Hall resistance Rfj = h/e^ is related to the fine-structure constant a, the dimensionless expansion

parameter of quantum electrodynamic (QED) theory, by

Rfj = HQc/2a. Here /io = 47r x 10"^ H/m is the magnetic permeability of vacuum and c = 299 792 458 m/s

is the speed of light in vacuum. Thus since /xq and c are exactly defined constants, if a is known from some QEDrelated experiment with a given uncertainty, R^ will be

known in SI units with the same uncertainty. An alternate method of determining R^ is to measure a Hall resistance

Rnii) in terms of QlaB' which in turn is measured in terms of the SI ohm by means of a calculable capacitor.

Most of the major national standards laboratories as well

as the BIPM are currently putting into place the apparatus

necessary to define and maintain their unit of resistance

using the QHE. Thus the CCE also plans to review all relevant experiments at its September 1986 meeting and

if justified, adopt a value of Rfj = h/e^ consistent with

the SI to be used by every national standards laboratory

(and BIPM) that chooses to employ the QHE to define and

maintain its laboratory unit of resistance [19], [20]. Since

for most countries Qlab difi'ers from the SI unit by only a few tenths to 2 ppm, and the number of laboratory and

industrial resistance standards and related instruments of

this level of precision is limited, the changeover to the

*The magnetic field is applied normal to the 2DEG, which is in the plane

of the device; a current is passed along the length of the device normal to the field and carried by the 2DEG; and the Hall voltage is measured in the

direction normal to the current across the width of the device.

QHE should have a milder impact than the adoption of a new value of 2e/h.

VI. Conclusion

Adopting a new value for the Josephson frequencyvoltage ratio 2e/h and a value for the quantized Hall re- sistance Rff = h/e^, both consistent with the SI and both

universally accepted and used to define national units of

voltage and resistance, would clearly represent a major

advance for the international compatibility of electrical measurements and their conformity with the internationally accepted system of units, the SI. However, the lesson

of what happened in 1968, when VgiPM was changed by

1 1 ppm, and the principal point of this paper is clear: no

recommendations regarding values of2e/h and R^ should

be made unless the data and their uncertainties warrant it, and these uncertainties must be evaluated with great care and objectivity. Indeed, because changing the as-maintained electrical units will have an enormous impact on

our technology-dependent industrialized society, one

might argue that no decisions should be made until the uncertainties for both 2e/h and Rff are conservatively es- timated to be at the 0.1-0.2-ppm level. This would effectively guarantee that no further changes would be necessary for many decades, if at all. See [32] for a summary

of the September 1986 meeting of the CCE.

References

[1] J. C. Gallop, "The impact of superconducting devices on precision

metrology and fundamental constants," Metrologia, vol. 18, no. 2, pp. 67-92, 1982.

[2] Com. Intl. Poids Mes. Com. Consult. d'Electricite, Trav. ly Session, Oct. 1972, pp. E13-E14.

[3] J. Terrien, "News from the Bureau International des Poids et Mesures," Metrologia, vol. 9, no. 1, pp. 40-43, 1973.

[4] P. V. Seances Com. Intl. Poids Mes., 61' Session, vol. 40, Oct. 1972.

pp. 22, 100.

[5] Com. Intl. Poids Mes. Com. Consult. d'Electricite, Trav. 14' Session, May 1975, pp. E6-E7.

[6] J. Terrien, "News from the Bureau International des Poids et Mesures," Metrologia, vol. 11, no. 4, pp. 179-183, 1975.

[7] P. V. Seances Com. Intl. Poids Mes., 64' Session, vol. 43, MayJune 1975, pp. 10, E7.

[8] P. V. Seances Com. Intl. Poids Mes., 65' Session, vol. 44, Sept. 1976, pp. 59-60.

[9] G. Leclerc and T. J. Witt, "The conservation and the dissemination

of the volt by the BIPM since 1932," Euromeas 77, Precise Electrical Measurement, lEE Conf. Publ. No. 152, London, 1977, pp. 61-63.

[10] J. J. Denton, "Valeur numerique de la constante 2e /h," Com. Intl. Proc. Poids Mes. Com. Consult. d'Electricite, Trav. 13' Session, Oct. 1972, pp. E75-E76.

[11] E. R. Cohen and B. N. Taylor, "The 1973 least squares adjustment

of the fundamental constants," J. Phys. Chem. Ref. Data, vol. 2, no.

4, pp. 663-734, 1973.

[12] E. R. Cohen and B. N. Taylor, "The 1986 CODATA recommended

values of the fundamental physical constants," J. Res. Natl. Bur.

Stand., vol. 92, no. 2, pp. 85-95, Mar.-Apr. 1987.

[13] B. N. Taylor, "Is the present realization of the absolute ampere in error?" Metrologia, vol. 12, no. 2, pp. 81-83, 1976.

[14] B. P. Kibble, R. C. Smith, and I. A. Robinson, "The NPL movingcoil ampere determination," IEEE Trans. Instrum. Meas., vol. IM32, no. 1, pp. 141-143, Mar. 1983.

[15] G. J. Sloggett, W. K. Clothier, M. F. Currey, D. J. Benjamin, and

H. Baimsfather, "Absolute determination of the volt using a liquid electrometer," IEEE Trans. Instrum. Meas., vol. IM-34, no. 2. pp. 187-191, June 1985.
IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-36. NO. 2. JUNE 1987

[16] B. N. Taylor, "New results from previously reported fundamental

constant determinations," J. Res. Nat. Bur. Stands., vol. 90, no. 2, pp. 91-94, Mar. -Apr. 1985.

[17] G. Leclerc, "Etalons representatifs de I'ohm et du volt, situation au 1" Janvier 1977," Meirologia, vol. 14, no. 4, pp. 171-174, 1978;

also Erratum, Metrologia, vol. 15, no. 3, p. 116, 1979.

[18] V. I. Krzhimovskii, K. A. Ainitdinov, S. I. Borovitskii, and A. S. Gokhner, "Reproduction of the volt unit by means of long-lived nio- bium single-crystal Josephson junctions," Izmer. Tekh., vol. 19, no.

4. pp. 31-32, Feb. 1983 (English transl.: Meas. Tech., vol. 26, no.

2, pp. 123-126, July 1983.)

[19] Com. Intl. Poids Meas. Com. Consult. d'Electricite, Trav. 16" Session, p. E111-E112, El 16, March 1983.

[20] P. Giacomo, "News from the BIPM," Metrologia, vol. 20, no. 1, pp. 25-30, 1984.

[21] Com. Intl. Poids Mes. Com. Consult. d'Electricite, Trav. 12' Session, Oct. 1968, pp. E17-E18.

[22] J. Terrien, "News from the Bureau International des Poids et Mesures," Metrologia, vol. 5, no. 2, pp. 68-71, 1969.

[23] P. V. Seances Com. Intl. Poids Meas., 57" Session, vol. 36, Oct. 1968, pp. 17, 101.

[24] Com. Intl. Poids Mes. Com. Consult. d'Electricite, Trav. 12" Session, Oct. 1968, p. E14.

[25] Com. Intl. Poids Mes. Com. Consult. d'Electricite, Trav. IT Session. May 1965, p. EI2.

[26] J. Terrien, "News from the Bureau International des Poids et Mesures," Metrologia, vol. 1, no. 3, pp. 133-134, 1965.

[27] J. Terrien, "The work of the Bureau International des Poids et Mesures concerning electromagnetic units and measurements," IEEE

Trans. Instrum. Meas., vol. IM-15, no. 4, pp. 140-145, Dec. 1966.

[28] B. N. Taylor, W. H. Parker, and D. N. Langenberg, "Determination

oi e/h using macroscopic quantum phase coherence in superconductors: Implications for quantum electrodynamics and the fundamental

physical constants," Rev. Mod. Phys., vol. 41, no. 3, pp. 375-496,

July 1969.

[29] A. M. Thompson, "An absolute determination of resistance based on

a calculable standard of capacitance," Metrologia, vol. 4, no. 1, pp. 1-7, 1968.

[30] K. V. Klitzing and G. Ebert, "Application of the quantum Hall effect

in metrology," Metrologia, vol. 21, no. 1, pp. 11-18, 1985.

[31] M. E. Cage, R. F. Dziuba, and B. F. Field, "A test of the quantum

Hall effect as a resistance standard," IEEE Trans. Instrum. Meas.,

vol. IM-34, no. 2, pp. 301-303, June 1985.

[32] B. N. Taylor, "Report on the 17th session of the Consultative Committee on Electricity," J. Res. Nat. Bur. Stand., vol. 92, no. 1, pp. 55-61, Jan. -Feb. 1987.

<^U . S

.

GOVERNMENT PRINTING F F I C E i 1 9 8 9- 2 4 2 -2 / 00 I 8
NBS-n4A (REV. 2-8C) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NIST/TN-1263

2. Performing Organ. Report NoJ 3. Publication Date

June 1989

4. TITLE AND SUBTITLE

Guidelines for Implementing the New Representations of the

Volt and Ohm Effective January 1, 1990

5. AUTHOR(S) Norman B. Beleckl, Ronald F. Dziuha, Bruce F. Field

and Barry N. Taylor

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL INSTTTUTE OF STANDARDS AND TECHNOLOGY

(formerly NATIONAL BUREAU OF STANDARDS)

U.S. DEPARTMENT OF COMMERCE

QAITHERSBURQ, MD 20899

7. Contract/Grant No.

8o Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Same as item #6

10. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here) This document provides general guidehnes and detailed instructions on how to bring

laboratory reference standards of voltage and resistance and related instrumentation

into conformity with newly established and internationally adopted representations of

the volt and ohm. Based on the Josephson and quantum Hall effects, respectively, the

new representations are to come into effect worldwide starting on January 1, 1990.

Their implementation in the U.S. will result in increases in the values of the national

volt and ohm representations maintained at the National Ir\stitute of Standards and

Technology (NIST, formerly the National Bureau of Standards or NBS) of 9.264 parts

per million (ppm) and 1.69 ppm, respectively. The resulting increase in the value of the

U.S. representation of the ampere will be 7.57 ppm and in the U.S. electrical representation of the watt, 16.84 ppm. Also discussed are the effects on electrical standards of

the January 1, 1990, replacement of the International Practical Temperature Scale of

1968 by the International Temperature Scale of 1990, and of the January 1, 1990, approximate 0.14 ppm decrease in the U.S. representation of the farad.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

1990 change in electrical units; ohm maintenance; volt maintenance

13. AVAILABILITY

[XI Unlimited

r~l For Official Distribution. Do Not Release to NTIS

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